I have done it multiple times for different versions of Java with nominal effort. Of course, difficulty may vary depending on a project.

With projects like OpenRewrite [1] and good LLMs, things are a lot easier these days.

[1]: https://docs.openrewrite.org

Yes. I moved a few repository from java 8 up to Java 21.

Java 8 -> 9 is the largest source of annoyances, past that it's essentially painless.

You just change a line (the version of the JRE) and you get a faster JVM with better GC.

And with ZGC nowadays garbage collection is essentially a solved problem.

I worked on a piece of software serving almost 5 million requests per second on a single (albeit fairly large) box off a single JVM and I was still seeing GC pauses below the single millisecond (~800 usec p99 stop the world pauses) despite the very high allocation rate (~60gb/sec).

The JVM is a marvel of software engineering.