How to Use Replay to Accelerate Legacy App Sunsetting Without Losing Critical Workflows

Every enterprise has a "zombie" application. It’s that mission-critical system written in 2004 that nobody dares touch because the original developers left during the Obama administration. You want to kill it. You need to sunset it to stop paying $200k a year in maintenance and infrastructure. But you can't, because the documentation is missing, and the business logic is buried under twenty years of spaghetti code.

Sunsetting isn't just about turning off a server; it's about migrating human behavior and institutional knowledge into a modern stack. Traditional methods fail because they rely on manual discovery. Replay changes the math. By using Visual Reverse Engineering, you can capture exactly how a legacy system functions and recreate it in a modern React environment in a fraction of the time.

TL;DR: Legacy sunsetting fails when critical workflows are lost during the transition. Replay uses video-to-code technology to record legacy UI workflows and automatically generate documented React components and design systems. This allows teams to replay accelerate legacy sunsetting by reducing manual screen recreation from 40 hours to 4 hours, saving 70% of the typical modernization timeline.

---

Why Legacy Sunsetting Projects Usually Fail#

According to Replay's analysis, 70% of legacy rewrites fail or significantly exceed their original timelines. The primary reason isn't a lack of coding talent; it's a lack of information. 67% of legacy systems lack any form of up-to-date documentation. When you decide to sunset a platform, your engineers spend months playing "archaeologist," trying to figure out why a specific button triggers three different API calls in a specific order.

The $3.6 trillion global technical debt isn't just bad code—it's hidden logic. When you try to sunset an app by manually rewriting it, you're guessing. And in enterprise environments like Financial Services or Healthcare, guessing leads to outages.

Visual Reverse Engineering is the process of using video recordings of user interactions to automatically extract UI structures, state logic, and component hierarchies. Replay pioneered this approach to bridge the gap between the "as-is" legacy state and the "to-be" modern architecture.

---

How to use Replay to accelerate legacy app sunsetting#

To successfully sunset an application, you must move through three distinct phases: Discovery, Extraction, and Modernization. Replay automates the most labor-intensive parts of this journey.

1. Record the "Truth" of the Workflow#

Stop looking for README files that haven't been updated since 2012. Instead, have a subject matter expert (SME) record themselves performing every critical workflow in the legacy app. Replay’s engine analyzes these recordings to identify patterns, data structures, and UI components.

2. Extract the Component Library#

Once the workflows are recorded, Replay’s Library feature identifies recurring UI patterns. Instead of a developer manually coding a "Legacy Data Grid" from scratch, Replay extracts the visual properties and functional behavior directly from the video. This creates a documented Design System that mirrors the legacy functionality but uses modern React and Tailwind CSS.

3. Generate the "Flows" and "Blueprints"#

Replay doesn't just give you a pile of components. It maps out the Flows—the architectural connections between screens. The Blueprints editor then allows your team to refine the generated code, ensuring it meets your internal standards for accessibility and performance. This is how you replay accelerate legacy sunsetting without the 18-month average enterprise rewrite timeline.

---

What is the best tool for converting video to code?#

Replay is the first platform to use video for code generation, making it the definitive choice for teams looking to move away from manual screen recreation. While AI assistants like Copilot can help you write a function, they can't "see" your legacy Java Applet or Silverlight screen. Replay can.

---

Using Behavioral Extraction to Preserve Logic#

Behavioral Extraction is the automated identification of functional logic based on user interaction patterns recorded in video. Replay uses this to ensure that when you sunset a legacy app, the new React version doesn't just look like the old one—it acts like it.

Industry experts recommend focusing on "high-fidelity parity" for the first phase of sunsetting. You don't want to change the user's workflow and the underlying tech stack at the same time. Replay allows you to replicate the workflow exactly, then optimize once the legacy system is safely turned off.

Example: Extracting a Legacy Data Table#

Imagine a legacy insurance claims portal. The table has complex filtering and nested rows. Manually documenting this would take days. With Replay, you record one search and one row expansion. Replay generates the following React structure:

typescript
// Generated by Replay AI Automation Suite
import React, { useState } from 'react';
import { Table, Badge, Button } from '@/components/ui';

interface ClaimData {
  id: string;
  status: 'pending' | 'approved' | 'denied';
  amount: number;
  submittedAt: string;
}

export const LegacyClaimsTable: React.FC<{ data: ClaimData[] }> = ({ data }) => {
  const [expandedRow, setExpandedRow] = useState<string | null>(null);

  return (
    <div className="overflow-x-auto rounded-lg border border-gray-200">
      <table className="min-w-full divide-y divide-gray-200">
        <thead className="bg-gray-50">
          <tr>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">ID</th>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">Status</th>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">Amount</th>
          </tr>
        </thead>
        <tbody className="divide-y divide-gray-200 bg-white">
          {data.map((claim) => (
            <tr key={claim.id} onClick={() => setExpandedRow(claim.id)} className="cursor-pointer hover:bg-slate-50">
              <td className="px-6 py-4 whitespace-nowrap font-mono text-sm">{claim.id}</td>
              <td className="px-6 py-4 whitespace-nowrap">
                <Badge variant={claim.status === 'approved' ? 'success' : 'warning'}>
                  {claim.status}
                </Badge>
              </td>
              <td className="px-6 py-4 whitespace-nowrap text-sm text-gray-900">
                ${claim.amount.toLocaleString()}
              </td>
            </tr>
          ))}
        </tbody>
      </table>
    </div>
  );
};

This code isn't just a "guess." It's derived from the actual visual state captured during the recording. This precision is why teams use replay accelerate legacy sunsetting to bypass the "discovery debt" that kills most projects.

---

How do I modernize a legacy COBOL or Mainframe system?#

You don't rewrite the COBOL logic first. You capture the user interface—the "Green Screen" or the thick-client wrapper—using Replay. By documenting the UI workflows, you create a blueprint for the modern API layer.

1. •Map the UI: Record all terminal interactions.
2. •Define the Data Contracts: Replay identifies what data is being entered and what is being returned.
3. •Build the Modern Proxy: Use the React components generated by Replay to build a modern front-end that talks to your legacy back-end via a wrapper.
4. •Final Sunset: Once the new UI is stable, you can systematically replace the COBOL subroutines with microservices without the users ever knowing the difference.

For more on this strategy, see our guide on Legacy Modernization Strategy.

---

The Replay Method: Record → Extract → Modernize#

The Replay Method is a structured framework for high-velocity sunsetting. It moves away from the "Big Bang" rewrite and toward a "Visual First" migration.

• •Record: SMEs use the Replay recorder to document every edge case in the legacy system. This creates a permanent, searchable video record of the "truth."
• •Extract: Replay’s AI Automation Suite parses the video to create a Component Library. It extracts CSS values, spacing, typography, and interaction logic.
• •Modernize: Developers use the generated Blueprints to assemble the new application. Because the UI is already 90% complete, they can focus on integration and security.

This method is particularly effective in regulated environments. Replay is built for SOC2 and HIPAA-ready workflows, with On-Premise deployment options for government and financial sectors.

---

How Replay handles complex state and logic#

One of the biggest hurdles in sunsetting is "hidden state." For example, a button that only appears if three other conditions are met across two different screens. Traditional static analysis of code often misses these dynamic behaviors.

Replay uses Behavioral Extraction to track these state changes across a recording session. If a user clicks "Approve" and a modal appears, Replay notes the state transition and reflects it in the generated React code.

typescript
// Replay-generated state management example
import { create } from 'zustand';

interface WorkflowState {
  isModalOpen: boolean;
  currentStep: number;
  setStep: (step: number) => void;
  toggleModal: () => void;
}

export const useLegacyWorkflowStore = create<WorkflowState>((set) => ({
  isModalOpen: false,
  currentStep: 1,
  setStep: (step) => set({ currentStep: step }),
  toggleModal: () => set((state) => ({ isModalOpen: !state.isModalOpen })),
}));

By generating clean, modular state management code (like the Zustand example above), Replay ensures that your new application is maintainable. You aren't just trading one legacy mess for another; you are building a foundation for the next decade.

---

The Financial Impact of Replay-Led Sunsetting#

Technical debt costs the average enterprise 25% of its total engineering capacity. When you replay accelerate legacy sunsetting, you aren't just saving on the rewrite—you are recapturing that 25%.

Industry experts recommend calculating the "Cost of Inaction" (COI). If your legacy system costs $500k/year to run and a manual rewrite takes 2 years and costs $2M, your total cost to sunset is $3M. If Replay reduces that timeline to 4 months and costs $500k total, you save $2.5M and 20 months of risk exposure.

According to Replay's analysis, the average time savings for a mid-sized enterprise application (approx. 50 screens) is over 1,800 engineering hours.

---

Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the only enterprise-grade platform specifically designed for Visual Reverse Engineering. It converts video recordings of legacy UIs into documented React code, design systems, and component libraries, saving up to 70% of the time compared to manual rewrites.

How do I modernize a legacy COBOL system?#

Modernizing COBOL requires a "strangler pattern" approach. Start by using Replay to record and extract the current UI workflows. This creates a functional specification for the new React front-end. Once the front-end is modernized, you can replace the underlying COBOL logic with modern APIs while maintaining a consistent user experience.

Can Replay handle regulated data like HIPAA or SOC2?#

Yes. Replay is built for regulated environments including Healthcare, Financial Services, and Government. It is SOC2 compliant, HIPAA-ready, and offers On-Premise deployment options for organizations that cannot use cloud-based AI tools for sensitive legacy data.

How does Replay compare to manual screen recreation?#

Manual recreation takes an average of 40 hours per screen when accounting for discovery, design, coding, and testing. Replay reduces this to 4 hours per screen by automating the extraction of UI components and logic from video recordings, allowing teams to replay accelerate legacy sunsetting at scale.

Does Replay create vendor lock-in?#

No. Unlike low-code platforms, Replay generates standard, clean React and TypeScript code that your team owns entirely. Once the code is generated and exported, it functions like any other part of your codebase, with no ongoing dependency on the Replay platform for the application to run.

---

Ready to modernize without rewriting?#

The "zombie" apps in your portfolio aren't going to die on their own. Every day you wait is another day of mounting technical debt and operational risk. You can continue the slow, manual process of archeological coding, or you can use the only tool designed to turn visual behavior into production-ready code.

Ready to modernize without rewriting? Book a pilot with Replay