Many teams are still running Spring Boot 2.7.x. Spring Boot 2.x reached end of life in November 2023, which means no more security patches. The jump to 4.0 is two generations, and the breaking changes are real — but they are also well-documented and mostly automatable. This guide walks through the migration in stages: 2.x → 3.0 first, then 3.x incremental updates, then 4.0. Each section lists what breaks and how to fix it.