Migrating a large-scale Next.js monorepo from version 14 to 16 requires a strategic, phased approach to manage complexity and minimize production risk. This real-world case study details a 2-week migration of an enterprise application with 240+ route files and 3,000+ i18n strings. The process involved updating dependencies incrementally, migrating to named React imports, and handling the most significant breaking change: async route parameters.

The team leveraged Next.js codemods for standard pattern transformations and AI-assisted refactoring for context-aware changes across hundreds of files. Key steps included updating TypeScript configuration for the new JSX transform, adjusting monorepo peer dependencies for React 19 compatibility, and converting route parameters to type-safe strings. The migration resulted in a 50-60% faster build time and a 3% smaller bundle size.

By combining automated tools with a rigorous testing loop, the team completed the upgrade efficiently while maintaining code quality and business logic integrity.