Enterprise Legacy Migration

Case Study — Enterprise SaaS Platform

The Challenge

A US-based enterprise SaaS company had a legacy ASP application with injected inline scripts serving enterprise customers. The codebase had grown organically over 10+ years, making it increasingly difficult to maintain, test, and ship new features. Downtime and regressions were not an option — the product served paying enterprise clients with SLA requirements.

The Approach

Rather than a risky big-bang rewrite, I designed an incremental migration strategy:

1. Assessment — Mapped the entire codebase, identified coupling points, and prioritized migration paths by business impact
2. Bridge Layer — Built a compatibility layer allowing new components to coexist with legacy code, enabling parallel development
3. Incremental Migration — Migrated page-by-page from inline scripts to a fully bundled Vite + component-based architecture
4. Testing — Implemented comprehensive test coverage at each stage to catch regressions before they reached production
5. Tooling — Built custom migration tooling to automate repetitive patterns and reduce human error

Code: Before & After

A typical page migration — from inline scripts to a component-based architecture:

<script>
  var grid = document.getElementById('data-grid');
  $.ajax('/api/records', {
    success: function(data) {
      data.forEach(function(row) {
        var tr = document.createElement('tr');
        tr.textContent = row.name + ' | ' + row.status;
        tr.onclick = function() {
          openModal(row.id);
        };
        grid.appendChild(tr);
      });
    }
  });
  function openModal(id) { /* 200 lines of DOM manipulation */ }
</script>

<script setup lang="ts">
import { ref, onMounted } from 'vue'
import { useRecords } from '@/composables/useRecords'
import DataGrid from '@/components/DataGrid.vue'
import RecordModal from '@/components/RecordModal.vue'

const { records, loading, fetch } = useRecords()
const selectedId = ref<string | null>(null)

onMounted(() => fetch())
</script>

<template>
  <DataGrid
    :data="records"
    :loading="loading"
    @row-click="(row) => selectedId = row.id"
  />
  <RecordModal
    v-if="selectedId"
    :id="selectedId"
    @close="selectedId = null"
  />
</template>

AI Integration

In parallel with the migration, I architected the company's AI integration from scratch: RAG pipelines for domain-specific knowledge retrieval, MCP server configuration, Claude-powered features with structured prompting, and LLM-driven workflows now running in production serving real users.

Results

Key Takeaways

• Big-bang rewrites are almost never the right answer for production systems
• Migration tooling pays for itself within the first month
• Incremental delivery builds team confidence and catches issues early
• AI integration doesn't have to be a separate initiative — it can happen alongside modernization