You're going to need to give exemples here. Upgrading has been super simple.

The one time you actually may have had to work on things was with PHP 7, but that's because they finally removed things that had been deprecated for years, some for over a decade, and all of them were security risks or massive code smell anyway.

The one thing with PHP 8 was the removal of dynamic propery, and again that's a massive code smell that was holding the language back. Any code still using it when PHP 8 released was either written over a decade ago, or something that has way more issues than that with upgrading.