How to Modernize Legacy Underwriting Engines Without Losing Business Logic

70% of legacy rewrites fail or exceed their timelines. In the insurance and financial services sectors, that failure rate is often higher because the "source of truth" for underwriting logic isn't in a requirements document—it's buried in the undocumented behavior of a 20-year-old system.

When you attempt to modernize legacy underwriting engines using traditional "big bang" rewrites, you aren't just fighting technical debt; you are fighting the loss of institutional knowledge. With 67% of legacy systems lacking any up-to-date documentation, your engineers are essentially performing digital archaeology, trying to figure out why a specific risk multiplier was added in 2008.

The future of modernization isn't rewriting from scratch; it’s understanding and extracting what you already have through Visual Reverse Engineering.

TL;DR: Modernizing legacy underwriting engines fails when teams rely on manual documentation; Replay uses Visual Reverse Engineering to extract business logic, API contracts, and React components directly from user workflows, reducing modernization timelines from years to weeks.

The High Cost of the "Black Box" Underwriting Engine#

Underwriting engines are the most complex assets in the enterprise portfolio. They are often "black boxes" where input goes in, a decision comes out, and no one currently on the payroll knows exactly why. This lack of transparency contributes heavily to the $3.6 trillion global technical debt.

Traditional modernization efforts typically follow one of three paths, all of which carry significant risk:

The primary pain point is the "Documentation Gap." When you ask a developer to modernize a legacy screen, they spend an average of 40 hours per screen just trying to map out the dependencies and business rules. With Replay, that time is slashed to 4 hours because the platform uses the video of the workflow as the source of truth.

Why Manual Documentation is a Death March#

Most Enterprise Architects start a modernization project by hiring a fleet of consultants to "document the system." This is a mistake.

1. •Stale Data: Documentation is outdated the moment it is written.
2. •Hidden Rules: Legacy code often contains "if/else" chains that only trigger under hyper-specific edge cases that consultants will never find in a code review.
3. •The "Why" vs. "What": Code tells you what happens; user behavior tells you why it matters for the business process.

⚠️ Warning: Attempting to modernize an underwriting engine without a verified map of current state business logic is the leading cause of "Logic Drift," where the new system produces different financial outcomes than the legacy system.

The Replay Methodology: Visual Reverse Engineering#

Instead of digging through COBOL or legacy Java files, Replay records real user workflows. By observing how an underwriter interacts with the legacy system, Replay’s AI Automation Suite identifies the components, the data state changes, and the API calls required to replicate that functionality in a modern stack.

Step 1: Workflow Recording#

An expert user performs a standard underwriting task—for example, processing a high-value life insurance application. Replay captures every click, every state change, and every network request.

Step 2: Component Extraction#

Replay’s "Blueprints" editor analyzes the recording and generates documented React components. This isn't just "spaghetti code" generation; it’s clean, modular code that follows your organization's Design System (stored in the Replay Library).

Step 3: Logic & Contract Generation#

The platform identifies the business logic embedded in the UI. If a field becomes "Read-Only" when a certain risk threshold is met, Replay captures that rule and generates the corresponding API contract and E2E tests.

Step 4: Technical Debt Audit#

Before a single line of new code is deployed, Replay provides a Technical Debt Audit, highlighting which parts of the legacy logic are redundant and which are critical for the new system.

Technical Implementation: From Video to React#

To modernize legacy underwriting, you need more than just a UI facelift. You need a functional equivalent that integrates with modern microservices. Replay generates copy-paste ready code that preserves the business logic while moving it to a modern architecture.

Below is an example of a generated React component that Replay might extract from a legacy underwriting screen, including the preserved validation logic.

typescript
// Generated by Replay - Legacy Underwriting Module: RiskAssessment
import React, { useState, useEffect } from 'react';
import { TextField, Alert, Button } from '@your-org/design-system';

interface UnderwritingData {
  applicantAge: number;
  coverageAmount: number;
  hasPreExistingConditions: boolean;
}

export const RiskValidationComponent = ({ initialData }: { initialData: UnderwritingData }) => {
  const [data, setData] = useState(initialData);
  const [riskScore, setRiskScore] = useState<number>(0);

  // Replay extracted this logic from legacy workflow ID: UW-9928
  // Logic: If age > 60 and coverage > 1M, trigger mandatory manual review
  const calculateRisk = (age: number, coverage: number): number => {
    let score = (coverage / 100000) * (age / 10);
    if (age > 60 && coverage > 1000000) {
      console.warn("Legacy Rule Triggered: Mandatory Manual Review Required");
      return score * 1.5; 
    }
    return score;
  };

  useEffect(() => {
    setRiskScore(calculateRisk(data.applicantAge, data.coverageAmount));
  }, [data]);

  return (
    <div className="p-4 border rounded-lg shadow-sm">
      <h3 className="text-lg font-bold">Risk Assessment Module</h3>
      <TextField 
        label="Coverage Amount" 
        value={data.coverageAmount} 
        onChange={(val) => setData({...data, coverageAmount: Number(val)})} 
      />
      {riskScore > 50 && (
        <Alert severity="warning">
          High Risk Detected: This application requires senior underwriter approval.
        </Alert>
      )}
      <div className="mt-4">
        <strong>Calculated Risk Score:</strong> {riskScore.toFixed(2)}
      </div>
    </div>
  );
};

💡 Pro Tip: When using Replay, focus on your "Happy Path" workflows first. These represent 80% of your business value. Use the AI Automation Suite to identify the edge cases that are rarely triggered but carry the highest regulatory risk.

Preserving the API Contract#

One of the biggest hurdles in modernization is ensuring the new frontend can still communicate with the legacy backend (or a newly refactored microservice). Replay automatically generates API contracts based on the data observed during the recording phase.

json
{
  "contractName": "UnderwritingSubmission",
  "version": "2.0.1",
  "observedFields": [
    {
      "name": "policy_id",
      "type": "UUID",
      "required": true
    },
    {
      "name": "risk_multiplier",
      "type": "Float",
      "validation": ">= 1.0",
      "source": "Legacy_Calc_Engine_V4"
    }
  ],
  "endpoint_mapping": "/api/v1/underwriting/submit"
}

By generating these contracts, Replay ensures that your modernization effort doesn't break downstream dependencies in your core banking or insurance systems.

Built for Regulated Environments#

Underwriting is a highly regulated activity. Whether you are in Financial Services, Healthcare, or Insurance, you cannot afford "black box" AI or unverified code.

• •SOC2 & HIPAA Ready: Replay is built with the security requirements of the enterprise in mind.
• •On-Premise Availability: For organizations with strict data residency requirements (Government, Telecom), Replay can be deployed entirely within your own firewall.
• •Audit Trails: Every component generated by Replay is linked back to the original video recording, providing a clear audit trail for regulators to see exactly how the logic was derived.

💰 ROI Insight: A major Tier-1 bank used Replay to modernize their commercial loan underwriting portal. They reduced their estimated rewrite timeline from 18 months to just 12 weeks, saving an estimated $2.4M in developer hours.

The Step-by-Step Modernization Path#

If you are a CTO or Enterprise Architect tasked with modernizing a legacy underwriting engine, follow this battle-tested framework:

Step 1: Inventory and Prioritization#

Use Replay to catalog your existing screens. Identify which modules are "High Complexity / High Value" (e.g., Risk Calculation) and which are "Low Complexity / Low Value" (e.g., Address Updates).

Step 2: Capture the "Gold Standard" Workflow#

Have your most experienced underwriters record their sessions. This captures not just the code's intent, but the actual business process, including workarounds they've developed over years of using suboptimal software.

Step 3: Automated Extraction#

Run the recordings through Replay’s Blueprints. Generate the React components and the technical debt audit. This identifies which parts of the legacy system are actually being used and which 30% of the code can be safely retired.

Step 4: Integration and E2E Testing#

Use the generated E2E tests to ensure that the new modern components produce the exact same outputs as the legacy system. This "Parity Testing" is critical for regulatory compliance.

Step 5: Deployment#

Deploy the modernized modules using a Strangler Fig approach, replacing legacy screens one by one until the entire engine is running on a modern, documented React/Node.js stack.

Frequently Asked Questions#

How long does legacy extraction take?#

While a manual rewrite takes 18-24 months, Replay typically extracts a fully documented, functional component library and flow architecture in 2 to 8 weeks, depending on the number of screens.

What about business logic preservation?#

This is Replay's core strength. By using "Video as the source of truth," we don't just guess what the code does; we see what the system actually does. We extract the state transitions and validation rules directly from the runtime behavior, ensuring 100% logic parity.

Does Replay work with mainframe or terminal-based systems?#

Yes. As long as there is a visual interface (even a "green screen" terminal emulator), Replay can record the workflow and extract the underlying data patterns and logic to move it into a modern web-based environment.

How does this affect our current technical debt?#

Replay provides a comprehensive Technical Debt Audit. By identifying unused features and redundant logic during the extraction phase, you can reduce your codebase size by up to 40%, making future maintenance significantly cheaper.

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

The $3.6 trillion technical debt crisis isn't a coding problem; it's an understanding problem. We keep rewriting systems because we don't understand the ones we have. Replay changes that paradigm by turning the "black box" of legacy underwriting into a documented, modern codebase in a fraction of the time.

Stop guessing what your legacy code does. Start seeing it.

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