Build vs Buy for Modernization Tooling: Why Custom Migration Scripts Often Cost 5x More Than Expected

Your custom migration script is already technical debt.

The moment your team writes the first line of a "quick" scraper or a Python-based extraction utility to move logic from a 20-year-old legacy system to a modern stack, you have committed a cardinal sin of enterprise architecture: you’ve built a legacy system to migrate a legacy system.

Global technical debt is ballooning toward $3.6 trillion, yet 70% of enterprise rewrites still fail or exceed their timelines. Why? Because teams underestimate the "Archaeology Tax." They assume that because they have the source code, they understand the system. They don’t. In reality, 67% of legacy systems lack any meaningful documentation, leaving developers to guess at business logic buried in thousands of lines of spaghetti code.

The "Build vs Buy" debate for modernization tooling is often framed as a cost-saving exercise. In reality, building custom tooling for a "Big Bang" rewrite is the most expensive way to fail.

TL;DR: Custom migration scripts often cost 5x more than expected because they fail to capture undocumented business logic, require constant maintenance, and extend the "Big Bang" rewrite timeline to a dangerous 18-24 month window.

The Fallacy of the "Simple" Migration Script#

When faced with a monolithic legacy system—whether it’s a Silverlight application, a PowerBuilder monster, or a COBOL-backed green screen—the instinct of a Senior Developer is to "just write a script."

They believe they can parse the old UI, map the fields to a new API, and generate a React frontend. This approach ignores the reality of enterprise software. Legacy systems aren't just code; they are a collection of "hidden truths"—edge cases, regulatory patches, and undocumented workflows that only exist in the minds of users who have been using the system for a decade.

The Hidden Costs of Building Custom Tooling#

1. •The Maintenance Loop: Your custom migration tool needs its own CI/CD pipeline, its own unit tests, and its own bug fixes. You are now managing two projects instead of one.
2. •The Documentation Gap: Scripts can only extract what is explicitly defined in the code. They cannot capture the intent of a user workflow.
3. •The 40-Hour Screen Trap: On average, it takes 40 hours of manual effort to document, design, and code a single complex legacy screen into a modern framework.

💰 ROI Insight: Companies using Replay reduce this 40-hour manual process to just 4 hours per screen. Over a 100-screen application, that is a savings of 3,600 engineering hours.

Comparing Modernization Strategies#

When deciding between building custom scripts, using a traditional "Strangler Fig" approach, or leveraging Visual Reverse Engineering with Replay, the risk-to-reward ratio becomes clear.

The Technical Reality: Why Custom Scripts Fail#

Let's look at what a typical "custom script" output looks like versus a clean, reverse-engineered component. Most internal migration scripts produce "junk code"—tightly coupled, hard-to-test components that carry the sins of the old system into the new one.

Example: The "Custom Script" Mess#

This is what happens when a script tries to blindly map old logic to a new component. It’s unreadable and brittle.

typescript
// ❌ Problematic: Generated by a custom "quick" migration script
// Carries over legacy naming conventions and messy state management
export function LegacyUserForm_v2_FINAL() {
  const [state, setState] = useState({ 
    FLD_001: '', 
    is_valid_user_check_01: false 
  });

  // Business logic is a "black box" copied from the old system
  const handleSave = () => {
    if (state.FLD_001.length > 0 && window.legacyGlobalValidator(state.FLD_001)) {
      // Hardcoded API endpoints from the script
      fetch('/api/v1/old-endpoint/save', { method: 'POST', body: JSON.stringify(state) });
    }
  };

  return <div className="legacy-wrapper">...</div>;
}

Example: The Replay Extraction#

Replay doesn't just scrape code; it records real user workflows and uses AI to generate clean, documented React components and API contracts.

typescript
// ✅ Clean: Generated by Replay Visual Reverse Engineering
import { useForm } from '@/hooks/useForm';
import { Button, TextField } from '@/components/design-system';

/**
 * Extracted from: "User Registration Workflow"
 * Original System: Insurance Claims Portal (Legacy)
 * Business Logic: Validates policy number against regional constraints
 */
export function UserRegistrationForm() {
  const { register, handleSubmit, errors } = useForm({
    validationSchema: registrationSchema, // Generated API Contract
  });

  const onSubmit = async (data: RegistrationData) => {
    // Clean, modern async/await logic mapped to new backend
    await api.users.create(data);
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <TextField {...register('policyNumber')} label="Policy Number" error={errors.policyNumber} />
      <Button type="submit">Complete Registration</Button>
    </form>
  );
}

Moving From "Black Box" to Documented Codebase#

The core problem with legacy systems is that they are black boxes. You put data in, you get data out, and no one knows exactly what happens in the middle. Custom scripts treat the code as the source of truth. Replay treats the user’s interaction as the source of truth.

Step 1: Record the Workflow#

Instead of digging through thousands of lines of undocumented Java or C#, you simply record a subject matter expert (SME) performing the actual task in the legacy system. This captures the "happy path" and the edge cases that documentation misses.

Step 2: Visual Reverse Engineering#

Replay’s engine analyzes the recording. It identifies the UI components, the data structures being passed to the backend, and the business logic triggered by specific actions.

Step 3: Automated Extraction#

The platform generates:

• •Clean React Components: Using your organization's design system.
• •API Contracts: Standardized JSON schemas that your backend team can build against immediately.
• •E2E Tests: Automatically generated Playwright or Cypress tests based on the recorded workflow.

⚠️ Warning: If your modernization strategy doesn't include automated E2E test generation, you are setting yourself up for a regression nightmare that will haunt your "modern" system for years.

Built for the "No-Fail" Industries#

In Financial Services, Healthcare, and Government, a "failed rewrite" isn't just a budget overage—it's a regulatory disaster. These industries cannot afford the 18-month "Big Bang" risk.

Replay is designed for these high-stakes environments:

• •SOC2 & HIPAA Ready: Your data remains secure.
• •On-Premise Availability: For air-gapped or highly sensitive environments where cloud is not an option.
• •Technical Debt Audit: Before you write a single line of new code, Replay provides a full audit of your legacy system’s complexity.

📝 Note: Most custom scripts fail because they don't account for the security headers, session tokens, and complex authentication handshakes of legacy enterprise systems. Replay handles this natively during the extraction phase.

The Future Isn't Rewriting—It's Understanding#

The "Build" approach to modernization tooling is a relic of an era where we had more time than complexity. Today, the complexity of legacy systems has outpaced the ability of manual scripting to keep up.

By choosing a platform like Replay, you aren't just "buying a tool"—you are adopting a methodology that prioritizes understanding over guesswork. You are moving from "archaeology" to "architecture."

Frequently Asked Questions#

How long does legacy extraction take with Replay?#

While a manual rewrite of a single complex screen can take 40+ hours, Replay reduces this to approximately 4 hours. Most enterprise teams can move from a recorded workflow to a documented React component and API contract in a single afternoon.

What about business logic preservation?#

This is the primary failure point of custom scripts. Replay captures business logic by observing the state changes and API calls during a real user session. This ensures that even "hidden" logic—like a specific field validation that only triggers for users in a certain state—is documented and preserved in the generated code.

Does Replay require access to the original source code?#

No. Because Replay uses Visual Reverse Engineering, it works by observing the application's behavior and its communication with the backend. This is ideal for systems where the original source code is lost, undocumented, or written in languages your current team doesn't support.

Is the generated code "locked in" to the Replay platform?#

Absolutely not. Replay generates standard, high-quality TypeScript, React, and JSON. Once the extraction is complete, the code is yours to own, modify, and deploy anywhere. We provide the "bridge" to modernization, not a new cage.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.