Next.js 15 Upgrade Guide: App Router Changes, Caching Gotchas, and Safe Migration Steps

Table of Contents 1. Why upgrade now? 2. What changed in Next.js 15 (summary) 3. Core technical differences (quick reference) 4. Comparison: Pages Router (legacy) vs App Router (Next.js 15) 5. Common breaking issues and how they appear 6. Recommende...