There are two types of upgrades. One follows only `UPGRADE.md` files on every release, replacing what has been removed with new alternatives. It works, and we could say that the codebase will be "up-to-date." The other upgrade doesn't stop at the required minimum but **makes use of all modern features the framework provides**. It will be faster, easier to understand, and easier to upgrade to the next version. I [wrote a post](/blog/two-kinds-of-legacy-code-upgrade) that explains why the latter is better. There are no sources about Symfony upgrades spanning multiple major versions—time to fix that.