Post-Acquisition Integration: Unifying Legacy Frontends with React in 6 Months

The Board of Directors doesn't care about your technical debt; they care about "synergy realization." In the wake of a multi-billion dollar merger, the mandate is clear: unify the user experience, consolidate the brand, and eliminate redundant infrastructure. Yet, for the Enterprise Architect, the reality is a nightmare of disparate tech stacks. You are likely staring at a fragmented ecosystem where a 1990s mainframe green-screen sits alongside a 2012 jQuery monolith and a semi-modern Angular 2 app.

Traditional manual rewrites are a death sentence for M&A timelines. With a 70% failure rate for legacy rewrites and an average enterprise timeline of 18 months just to reach feature parity, the math simply doesn't work for a 6-month integration window. To succeed, you need a paradigm shift from manual reconstruction to Visual Reverse Engineering.

TL;DR: Post-acquisition integration often stalls due to undocumented legacy systems and massive technical debt. By leveraging Replay, enterprises can bypass the 18-month manual rewrite cycle, using video-to-code automation to extract UI logic and design tokens directly from legacy workflows. This reduces the time-per-screen from 40 hours to just 4 hours, enabling a unified React-based frontend in under 6 months.

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The $3.6 Trillion Bottleneck in M&A#

Post-acquisition integration unifying legacy systems is the single greatest challenge in modern corporate strategy. According to Replay’s analysis, the global technical debt currently sits at a staggering $3.6 trillion. When two companies merge, this debt isn't just added; it's compounded.

The primary friction point is documentation—or the lack thereof. Industry experts recommend a "documentation-first" approach, but 67% of legacy systems lack any form of updated documentation. This leaves engineers "archeology-mining" old codebases to understand business logic before they can even write a single line of React.

Video-to-code is the process of recording a user interacting with a legacy application and using AI-driven visual analysis to automatically generate documented React components, CSS modules, and state logic.

By using Replay, architects can stop guessing what a legacy button does and start generating the code that replicates its exact behavior in a modern framework. This is the only way to meet the aggressive "Day 2" integration goals set by executive leadership.

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The 6-Month Roadmap for Postacquisition Integration Unifying Legacy#

To move from a fragmented mess to a unified React frontend in two quarters, you cannot follow a standard SDLC. You need an accelerated, automated pipeline.

Phase 1: Audit and Discovery (Weeks 1-4)#

The first month is about mapping the "Surface Area of Integration." You cannot unify what you don't understand. Instead of reading through millions of lines of COBOL or legacy Java, use Replay's Flows feature to record every critical user journey in the acquired company’s software.

Phase 2: Design System Extraction (Weeks 5-8)#

In the second month, you must establish a "Single Source of Truth." This is where most integrations fail—trying to force a new brand onto old CSS. Replay’s Library automatically extracts design tokens (colors, typography, spacing) from recorded sessions to build a unified Design System.

Phase 3: Component Generation (Weeks 9-16)#

This is the execution phase. Using Replay's Blueprints, your team converts the recorded legacy screens into functional React components. This is where the 70% average time savings becomes visible. While a manual rewrite takes 40 hours per screen, Replay reduces this to 4 hours.

Phase 4: Data Integration and Testing (Weeks 17-24)#

With the UI unified in React, the final months focus on hooking these components into the new consolidated backend APIs and performing rigorous UAT in regulated environments.

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Technical Implementation: Bridging the Gap#

When performing a postacquisition integration unifying legacy frontend, you often need to "strangle" the old application. This means running the new React components alongside the legacy system during the transition.

Below is a conceptual example of how an architect might define a "Bridge Component" that allows a modern React UI to communicate with an underlying legacy state machine, a common requirement during the middle stages of a 6-month integration.

typescript
// LegacyBridge.tsx
import React, { useEffect, useState } from 'react';

interface LegacyProps {
  legacyModuleId: string;
  onActionComplete: (data: any) => void;
}

/**
 * During post-acquisition integration, we use Bridge Components
 * to wrap legacy functionality while the Replay-generated 
 * React components are being wired to new microservices.
 */
export const LegacyBridge: React.FC<LegacyProps> = ({ legacyModuleId, onActionComplete }) => {
  const [isLegacyLoaded, setLegacyLoaded] = useState(false);

  useEffect(() => {
    // Simulate connection to a legacy global state object (e.g., jQuery/Window)
    if (window.LegacyApp) {
      window.LegacyApp.initializeModule(legacyModuleId);
      setLegacyLoaded(true);
    }
  }, [legacyModuleId]);

  const handleLegacyTrigger = () => {
    const data = window.LegacyApp.getModuleData(legacyModuleId);
    onActionComplete(data);
  };

  return (
    <div className="modern-container">
      <h3>Integration Layer: {legacyModuleId}</h3>
      {isLegacyLoaded ? (
        <button onClick={handleLegacyTrigger} className="replay-primary-btn">
          Sync Legacy Data to React
        </button>
      ) : (
        <p>Loading Legacy Context...</p>
      )}
    </div>
  );
};

However, the goal is to move away from these wrappers as quickly as possible. Replay's AI Automation Suite allows you to bypass this "wrapper hell" by generating clean, independent React components that don't rely on the legacy window object.

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Comparison: Manual Rewrite vs. Replay Visual Reverse Engineering#

As the table illustrates, the efficiency gains aren't just incremental—they are transformative. For a firm handling a postacquisition integration unifying legacy assets across multiple business units, the cost savings alone can reach millions of dollars.

Learn more about building Design Systems from legacy code

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The "Flows" Methodology: Mapping the Acquisition#

One of the biggest risks in M&A is losing "tribal knowledge." When the original developers of the acquired company leave, the logic of the software is often lost. Replay mitigates this risk through its Flows feature.

Flows are interactive architectural maps generated from video recordings that document exactly how data moves through a legacy UI.

By recording a "Power User" performing their daily tasks, Replay captures the edge cases that are never documented in Jira tickets. The platform then translates these visual interactions into a structured React hierarchy.

Example: A Replay-Generated React Component#

Here is the type of clean, modular code Replay produces from a legacy recording of a financial ledger screen:

tsx
// ReplayGeneratedLedger.tsx
import React from 'react';
import { useTable } from '@/hooks/useTable';
import { Button } from '@/components/ui/design-system';

interface Transaction {
  id: string;
  amount: number;
  status: 'pending' | 'completed' | 'failed';
  timestamp: string;
}

/**
 * This component was generated via Replay Blueprints.
 * It maintains the logic of the legacy 'GL-700' screen 
 * but uses modern Tailwind CSS and React state.
 */
export const UnifiedLedger: React.FC = () => {
  const { data, loading } = useTable<Transaction>('/api/v1/transactions');

  if (loading) return <div className="animate-pulse">Loading Unified Ledger...</div>;

  return (
    <div className="p-6 bg-white rounded-lg shadow-md">
      <header className="flex justify-between mb-4">
        <h2 className="text-xl font-bold text-slate-800">Acquisition Integration Ledger</h2>
        <Button variant="outline">Export to CSV</Button>
      </header>
      <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">
          {data.map((row) => (
            <tr key={row.id}>
              <td className="px-6 py-4 whitespace-nowrap text-sm font-mono">{row.id}</td>
              <td className="px-6 py-4 whitespace-nowrap">
                <span className={`pill-${row.status}`}>{row.status}</span>
              </td>
              <td className="px-6 py-4 whitespace-nowrap text-sm text-gray-900">
                ${row.amount.toLocaleString()}
              </td>
            </tr>
          ))}
        </tbody>
      </table>
    </div>
  );
};

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Solving the Security and Compliance Hurdle#

In industries like Financial Services, Healthcare, and Government, postacquisition integration unifying legacy systems isn't just a technical challenge—it's a regulatory one. You cannot simply upload legacy screenshots to a public AI model.

Replay is built for these high-stakes environments. With SOC2 compliance, HIPAA-readiness, and On-Premise deployment options, Replay ensures that sensitive PII (Personally Identifiable Information) captured during the recording process remains within your secure perimeter.

According to Replay's analysis, enterprise security reviews are the #1 cause of delay in modernization projects. By providing an on-premise solution, Replay cuts the security approval process from months to weeks.

Read about modernizing Financial Services legacy stacks

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Why Manual Rewrites Fail During M&A#

The "18-month average enterprise rewrite timeline" is a optimistic estimate. In reality, most projects enter a state of "feature creep" where the new system is expected to do everything the old system did, plus everything the new board wants.

1. •The Documentation Gap: As mentioned, 67% of systems lack docs. Engineers spend 50% of their time just trying to understand the legacy code.
2. •The Talent Drain: Post-acquisition, key developers often leave. Without them, the manual rewrite loses its "map."
3. •The 40-Hour Screen: Manually identifying every state change, CSS hover effect, and validation rule on a single complex screen takes a full work week for a senior dev.

Replay solves these by making the UI itself the documentation. If it happens on the screen, Replay captures it and turns it into code. This is how you achieve a 70% time savings.

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Frequently Asked Questions#

How does Replay handle complex business logic hidden in legacy code?#

Replay’s Visual Reverse Engineering doesn't just look at the UI; it observes the state changes and data flows resulting from user interactions. While the backend logic remains in your APIs, Replay ensures the frontend logic—such as form validation, conditional rendering, and UI state—is perfectly replicated in the new React components.

Can we use Replay for on-premise legacy systems with no internet access?#

Yes. Replay offers an On-Premise deployment model specifically designed for highly regulated industries like Defense, Government, and Banking. This allows you to perform postacquisition integration unifying legacy systems without any data leaving your secure internal network.

Does Replay generate clean, maintainable React code?#

Absolutely. Replay doesn't output "spaghetti code." It uses your existing Design System tokens and follows modern React best practices, including TypeScript types, functional components, and modular CSS/Tailwind. The code is indistinguishable from code written by a Senior Frontend Engineer.

How does Replay handle different legacy technologies like Flash, Silverlight, or Mainframes?#

If it can be rendered in a browser or captured via a screen recording, Replay can process it. Our AI models are trained to recognize patterns across various UI eras, converting even the most archaic "green-screen" layouts into responsive, accessible React components.

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Conclusion: The New Standard for M&A Integration#

The traditional approach to postacquisition integration unifying legacy frontends is broken. You cannot expect 21st-century business agility using 20th-century manual coding workflows. The $3.6 trillion technical debt crisis requires a move toward automation.

By adopting Replay, Enterprise Architects can finally deliver on the promise of M&A synergies. You can move from a fragmented, multi-stack nightmare to a unified, high-performance React ecosystem in months, not years.

Ready to modernize without rewriting? Book a pilot with Replay