JDK 21 is a few years old now, so you were a few years behind. JDK 25 was released last year, which makes for the next stable LTS for a while. JDK 28 is expected in 2028, and these features are not going to be enabled by default for years after that.

Java is generally backwards compatible, so unless you're using fat frameworks that use shady internals or known-deprecated APIs, you should generally be fine immediately upgrading to the latest LTS, possibly even non-LTS versions if you have confidence in your stack.

What did you go from to get to 21?

Mostly just hit the LTSes is what we've been doing and since about 17 it's been a pretty easy process in general.

Protip: If you ditch lombok everything gets a lot easier.

How much stuff actually broke? I feel like you can treat Java upgrades as minor upgrades most of the time, maybe you have to rebuild a few binaries but I'd be shocked if it required significant code changes.