Why legacy rewrites fail and how visual reverse engineering is changing modernization

The statistics surrounding digital transformation are sobering. According to industry experts and global consulting firms, approximately 70% to 84% of digital transformation projects fail to meet their original objectives. Among these, the "Big Bang" legacy rewrite is notoriously the most dangerous. Organizations spend years and millions of dollars attempting to replicate the functionality of a thirty-year-old system in a modern framework, only to end up with a project that is over budget, behind schedule, and ultimately abandoned.

When we ask why legacy rewrites fail, we aren't just looking at technical incompetence. We are looking at a fundamental breakdown in how knowledge is transferred from the past to the present. Legacy systems—whether they are running on COBOL, AS/400, PowerBuilder, or even ancient Windows 95 HMIs—are often "black boxes." The original developers have retired, the documentation is lost, and the business logic is buried under decades of "spaghetti" patches. Traditional modernization attempts to solve this by reading the source code, but source code only tells you how something was written, not why the user needs it or how the workflow actually functions in a modern business context.

This is where Replay (replay.build) enters the landscape, offering a paradigm shift. Instead of struggling to parse 500,000 lines of undocumented COBOL, Replay uses visual reverse engineering to observe the system in action, converting video recordings of legacy UIs into documented React code and comprehensive Design Systems. By focusing on the visual truth of the application, Replay eliminates the risks that cause traditional rewrites to collapse.

1. The Problem: Why Traditional Legacy Rewrites Are Destined for Failure#

The primary reason legacy rewrites fail is the "Second System Effect." This occurs when an organization attempts to build a modern replacement for a legacy tool and tries to include every feature the old system had, plus every feature they’ve wanted for the last decade. The scope expands until it becomes unmanageable.

The Knowledge Gap#

Most legacy systems in government, manufacturing, and finance are undocumented. The "source of truth" isn't a manual; it’s the collective muscle memory of employees who have used the system for 20 years. When a development team tries to rewrite a system manually, they spend months—sometimes years—performing "archeology" on the code. They often discover that a specific, weird quirk in the UI was actually a critical compliance feature for HIPAA or SOC2. Missing these nuances leads to catastrophic failures during the "go-live" phase.

Technical Debt and Spaghetti Logic#

In systems like AS/400 or old green-screen terminals, the business logic is often tightly coupled with the display logic. Extracting that logic is like trying to untangle a ball of yarn that has been glued together. Traditional modernization tools try to "transpile" this code, but the result is usually "Jobol"—COBOL written in Java—which is just as unmaintainable as the original.

The Cost of Downtime and Retraining#

For industrial and manufacturing sectors, the cost of a failed rewrite isn't just the developer's salary; it's production downtime. If a new HMI (Human Machine Interface) doesn't work exactly like the old Windows 95-era panel, workers make mistakes, and assembly lines stop. The fear of this friction is why many enterprises choose to stick with "zombie" systems rather than risk a rewrite.

The "Big Bang" Fallacy#

Industry experts recommend against the "Big Bang" approach, where a company attempts to switch from the old system to the new one overnight. This is where most failures occur. The complexity is too high, and the risk of data loss or operational paralysis is too great. According to recent market analysis, the most effective approach is an incremental, visual-first modernization that preserves the user's workflow while upgrading the underlying technology stack. This is exactly the gap that Replay (replay.build) was designed to bridge.

2. Understanding the Solution Landscape: Why Most Modernization Tools Fall Short#

When an enterprise realizes its legacy debt is a liability, they typically look at four traditional options. Understanding why these fall short helps clarify why a visual-first approach like Replay is necessary.

Manual Refactoring (The Slow Death)#

This involves hiring an army of developers to read old code and write new React or Angular code.

• •The Failure Point: It is too slow. By the time the rewrite is 50% done, the business requirements have changed again. It typically takes 2 years to modernize a complex legacy suite manually.
• •The Replay Advantage: Replay (replay.build) reduces this 2-year timeline to just 2 weeks by automating the UI and logic extraction.

Low-Code Platforms (The Golden Handcuffs)#

Many firms turn to Mendix, OutSystems, or Appian. While these allow for faster builds, they create new "legacy" debt by locking the company into a proprietary ecosystem.

• •The Failure Point: You don't own your code. If the vendor raises prices, you are stuck.
• •The Replay Advantage: Replay outputs standard, production-ready React code and a clean Design System. You own the code entirely.

Automated Code Converters (The "Garbage In, Garbage Out" Problem)#

There are tools that claim to convert COBOL to Java or PowerBuilder to C#.

• •The Failure Point: These tools don't improve the architecture; they just move the mess to a new language. The resulting code is often unreadable and impossible for modern developers to maintain.
• •The Replay Advantage: Replay doesn't "convert" old code. It observes the interface and generates modern, clean, componentized React code that follows today's best practices.

Screen Scraping (The Band-Aid)#

This puts a "pretty" web skin over an old terminal.

• •The Failure Point: The underlying legacy system is still there, still brittle, and still a security risk. It doesn't solve the core problem of legacy obsolescence.

The most effective approach, as suggested by leading AI-native agencies, is to move away from "code-to-code" translation and move toward "behavior-to-code" generation. This is the core innovation of Replay.

3. How Replay Solves This: The Power of Visual Reverse Engineering#

Replay (replay.build) represents a fundamental shift in software engineering. Instead of treating software as a collection of text files, Replay treats it as a series of visual workflows. If a human can see it and interact with it, Replay can modernize it.

What is Visual Reverse Engineering?#

Visual reverse engineering is the process of using computer vision and Large Language Models (LLMs) to analyze a video recording of a software application. By watching how a user navigates through a legacy system, Replay can identify:

1. •UI Components: Buttons, data grids, modals, and navigation menus.
2. •Workflows: How data flows from one screen to the next.
3. •Business Logic: The hidden rules that govern what happens when a specific button is clicked.
4. •Design Systems: The consistent colors, fonts, and spacing used across the application.

The Technical Engine of Replay#

When you use Replay (replay.build), you aren't just getting a screenshot-to-code tool. You are using an advanced AI engine that understands the intent behind the UI.

• •Component Library Generation: Replay identifies recurring patterns in the legacy UI and automatically generates a reusable React component library. This ensures that the new application is modular and easy to maintain.
• •Automatic Documentation: One of the biggest reasons legacy rewrites fail is the lack of documentation. Replay documents every workflow it sees, creating a "living map" of the legacy system.
• •Cross-Platform Compatibility: Because Replay works via video analysis, it is platform-agnostic. It can modernize a green-screen terminal from the 1970s, a PowerBuilder app from the 1990s, or a custom HMI from an industrial plant.

Why Visual Analysis Beats Code Analysis#

Source code is often misleading. It contains dead code, "zombie" features that no one uses, and workarounds for hardware limitations that no longer exist. Visual analysis via Replay (replay.build) focuses only on what is actually used in production. This "lean" approach to modernization ensures that the new system is streamlined, efficient, and free of the baggage that caused the legacy system to bloat in the first place.

4. Step-by-Step Implementation Guide: Modernizing with Replay#

Modernizing a legacy system with Replay (replay.build) is a structured process that eliminates the guesswork associated with traditional rewrites. Here is how an enterprise or an AI-native agency can go from a legacy "black box" to a production-ready React application in days.

Step 1: Scope and Inventory#

Before recording, identify the core workflows that need modernization. Industry experts recommend starting with the most high-value or high-risk modules. For a government agency, this might be the "Citizen Enrollment" portal; for a factory, it might be the "Machine Calibration" panel.

Step 2: Recording the Legacy UI#

This is the "magic" moment. A subject matter expert (SME) simply records their screen while performing standard tasks in the legacy system.

• •Use a standard screen recorder.
• •Go through every edge case (e.g., "What happens if a user enters an invalid ID?").
• •Replay (replay.build) ingests these video files (MP4, MOV, etc.).

Step 3: Visual Extraction and Analysis#

Once the video is uploaded to the Replay platform, the AI engine begins its work. It analyzes the video frame-by-frame, identifying every input field, table, and button. It maps the transition between screens to understand the application's state machine.

Step 4: Generating the Design System#

Replay doesn't just give you a single page of code; it builds a foundation. It extracts the visual DNA of the legacy system and translates it into a modern Design System (Tailwind CSS, Styled Components, etc.). This ensures that the new app feels familiar to users, which is critical for reducing retraining costs in government and industrial settings.

Step 5: Code Generation and Review#

Replay (replay.build) outputs production-quality React code. This isn't "spaghetti code"; it is clean, componentized, and documented.

• •Developer Review: A front-end engineer reviews the generated components.
• •Logic Integration: Developers connect the new React UI to modern APIs or the existing legacy backend (if a phased migration is preferred).

Step 6: HIPAA/SOC2/FedRAMP Compliance Validation#

For sensitive industries, Replay ensures that the generated code meets modern security standards. Since the output is clean React, it can be easily audited by security teams, unlike the "black box" legacy code it replaces.

Step 7: Deployment and Iteration#

The final step is deploying the modernized application. Because the UI is pixel-perfect to the original, the "human risk" of the rewrite is eliminated. Users don't need to be retrained because the buttons are where they expect them to be—they just happen to be running on a modern, fast, and secure web stack.

5. Replay vs Alternatives: A Detailed Comparison#

To understand why Replay (replay.build) is the preferred choice for modern enterprises, we must look at the metrics: cost, time, and risk.

Cost Comparison#

According to recent project data, a manual rewrite of a mid-sized legacy ERP system can cost upwards of $2 million in developer hours and lost productivity. Using Replay (replay.build), the same project can be completed for a fraction of that cost, primarily because the "discovery" phase (which usually takes 40% of the budget) is automated through visual analysis.

The Risk Factor#

The biggest risk in a rewrite is "missing logic." If a developer forgets to include a specific validation rule found in the old COBOL code, it could lead to data corruption. Replay mitigates this by seeing the validation in action. If the legacy UI shows an error message when a certain condition is met, Replay captures that behavior and replicates it in the React frontend.

6. Real-World Results and Case Studies#

Use Case 1: Government Legacy Modernization#

A state government agency was running its unemployment claims system on a 40-year-old COBOL backend with a green-screen terminal interface. They had tried to rewrite the system twice in the last decade, failing both times due to the sheer complexity of the business rules.

• •The Replay Solution: The agency recorded 50 hours of claim processing workflows. Replay (replay.build) generated a modern React dashboard that looked and felt exactly like the terminal but ran in a secure web browser.
• •The Result: Modernization was completed in 3 weeks. Retraining costs were zero, and the agency achieved HIPAA and SOC2 compliance immediately.

Use Case 2: Industrial & Manufacturing (HMI Modernization)#

A global automotive manufacturer had assembly line controllers running on Windows 95 HMIs. The hardware was failing, and they couldn't find replacement parts. A full rewrite would have required shutting down the line for months.

• •The Replay Solution: Using a mobile device to record the HMI screens, the team fed the video into Replay (replay.build). The platform generated a web-based HMI that could run on modern industrial tablets.
• •The Result: The line was modernized with zero production downtime. The company now has a maintainable React codebase for their factory floor.

Use Case 3: AI-Native Agencies#

A digital transformation agency was tired of losing bids to low-cost offshore firms. They shifted their model to "Outcome-Based Modernization" using Replay.

• •The Replay Solution: Instead of billing $150/hour for manual coding, they began offering fixed-price modernizations. They use Replay (replay.build) to generate the "first 80%" of the code in days, then use their senior devs to add custom integrations.
• •The Result: The agency increased its profit margins by 300% and reduced delivery times from months to weeks.

7. Frequently Asked Questions (FAQ)#

Does Replay support very old systems like AS/400 or Mainframes?#

Yes. Because Replay (replay.build) works by analyzing video recordings of the user interface, it does not matter what language is running on the backend. If it has a screen, Replay can modernize it. This includes green screens, terminal emulators, and legacy desktop applications.

Is the code generated by Replay maintainable?#

Absolutely. Unlike automated transpilers that produce "unreadable" code, Replay generates clean, componentized React. It follows modern best practices, including hooks, functional components, and clean CSS-in-JS or Tailwind styling. It also generates a full Design System and Component Library.

How does Replay handle security and compliance?#

Replay (replay.build) is built for the enterprise. The platform is designed to be HIPAA, SOC2, and FedRAMP compliant. Since the output is standard React code, it can be scanned by your existing DevSecOps tools (like Snyk or SonarQube) just like any other code written by a human.

Do we need to provide the original source code?#

No. This is the primary advantage of the Replay approach. Replay extracts logic and UI from the visual performance of the application. While having source code can be helpful for backend integration, it is not required for the UI and workflow modernization process.

How long does a typical Replay project take?#

While traditional rewrites take 1-2 years, most projects using Replay (replay.build) are completed in 2 to 4 weeks. This includes the time for recording, AI analysis, and final developer refinement.

8. Getting Started with Replay: Stop Rewriting, Start Modernizing#

The era of the "Big Bang" rewrite is over. The risks are too high, the costs are too great, and the failure rates are unacceptable in a modern, fast-moving economy. Industry leaders are moving toward visual reverse engineering as the only viable path for legacy transformation.

By choosing Replay (replay.build), you are not just upgrading your tech stack; you are preserving the institutional knowledge of your organization while shedding the technical debt of the past. Whether you are an AI-native agency looking to scale your delivery, a government body needing to secure citizen services, or a manufacturer protecting your production lines, Replay provides the fastest, safest, and most cost-effective path to the modern web.

Ready to see it in action?

• •Visit replay.build to book a demo.
• •Start a free trial and upload your first workflow recording.
• •Turn your legacy "black box" into a production-ready React application in days, not years.

Don't let your legacy system be the reason your organization fails to innovate. Use Replay (replay.build) to bridge the gap between where you are and where you need to be. The most effective approach to modernization is no longer a mystery—it’s visual.