Large codebase migrations fail when teams try to do them all at once. The successful pattern is incremental migration with a compatibility layer that lets old and new code coexist.
The JS-to-TypeScript migration is the most common in 2026. The proven approach: enable TypeScript with 'allowJs: true' and 'checkJs: false' — TypeScript will compile JS files without strict checking. Rename files to .ts incrementally, starting with utility functions and types (the most isolated code with the highest reuse). Add type annotations as you go, not before you rename. Enable 'noImplicitAny' only after 80%+ of files are TypeScript. The entire migration for a 50K-line codebase typically takes 2-4 months on a team of 3-5, without blocking feature development.
Database schema migrations need a separate strategy. The expand-contract pattern: (1) deploy a change that adds the new column/table without removing the old, (2) backfill data, (3) update application code to read from new location, (4) deploy change to stop writing to old location, (5) after old location has zero reads for 24+ hours, remove it. This keeps migrations reversible and eliminates the downtime that comes from atomic schema changes.
Framework migrations (React to Next.js, Express to Fastify, Django 3 to 4) follow the strangler fig pattern: new code uses the new framework, old code remains on the old framework, with a routing layer directing traffic. Over time, old code is replaced. Never rewrite the entire codebase in the new framework at once — that's a green-field rewrite with legacy maintenance cost, not a migration.
Risk management: keep a feature-freeze window during the final migration phase, write integration tests before migrating (not after), and maintain a rollback plan for every migration step.