How to Use AI to Migrate a Legacy App in 2026 (jQuery → React, PHP → Node)

Quick Answer

Use the strangler-fig pattern: run old and new side by side, migrate one route/component at a time with AI, keep tests green throughout. Never attempt a big-bang rewrite — they fail 70% of the time.

• Incremental migration keeps the business running
• AI shines at 1-to-1 translation (jQuery to React hooks, PHP controllers to Express)
• The human's job is architecture decisions, not typing

What You'll Need

• Legacy source code with version control
• Target stack chosen (React, Next.js, FastAPI, etc.)
• Test harness or Playwright for visual regression
• Reverse proxy (Caddy/Nginx) to route between old and new

Steps

1. Audit the legacy app. Prompt: List all routes, external dependencies, and database queries in this codebase.
2. Pick the first slice. A non-critical, self-contained page. Marketing pages work well.
3. Migrate one component. Prompt: Convert this jQuery plugin to a React component using hooks and TypeScript.
4. Add tests for parity. Playwright visual snapshots catch most regressions.
5. Route traffic via proxy. Caddy: new routes go to Next.js, everything else to legacy.
6. Repeat per slice. Auth, dashboard, checkout — one PR each.
7. Database migration last. Keep one DB during transition, migrate only after UI complete.
8. Decommission old code. When traffic drops to zero, remove.

Common Mistakes

• Big-bang rewrite. 70% fail rate historically.
• Migrating the DB first. High risk, low reward early on.
• No visual regression tests. Styling drifts invisibly.
• Changing behavior during migration. Migrate with identical behavior, then improve.

Top Tools

Tool	Use
Cursor agent	Multi-file translation
Caddy	Reverse proxy splitting
Playwright	Visual regression
Claude Code	CLI bulk refactors
AST-grep	Deterministic syntax rewrites

FAQs

Can AI migrate Rails to Node? Yes — controllers translate well. ActiveRecord → Prisma is clean. Business logic needs human review.

What about AngularJS to Angular? Easier than AngularJS to React due to shared Angular patterns.

Does AI preserve i18n? Only if you ask. Provide the locale files upfront.

How long does a typical migration take? Mid-sized app (50k LOC): 3-6 months with a 2-person team + AI.

Can I keep the old DB schema? Yes, and often should. Migrate schema later in a separate project.

What about SEO during migration? Keep URLs identical or set up 301 redirects. AI can generate the redirect map.

Conclusion

Legacy migration is the highest-value AI workflow of 2026. Strangler-fig + AI cut typical 18-month migrations to 6 months. Start with one page today. Misar Dev automates much of the translation step.