Migrating to Next.js 15 App Router: Lessons Learned

Next.js 15 introduces critical core changes—most notably, treating `params` and `searchParams` in pages as asynchronous Promises. While this improves compiler capability and makes component trees more flexible, it requires active refactoring of synchronous lookups. Additionally, modern cacheLife rules replace older revalidation conventions, forcing teams to audit caching architectures. We walk through a real-world multi-page application migration, outlining how to streamline server data-fetching and prevent layout reflows.