I've been involved in bringing real old Java 1.4 and 6 and whatnot up to 17 and from classic app servers into cloud, can take a bit of work but it's pretty straightforward, mostly switching out deprecated methods to their successors and copying over boilerplate config from similar applications.

Depends on the frameworks in use. I have done the same as you in bringing code that was originally started on 1.4 up to 17 and now 21 and it just took grunt work, but not too crazy since most of it was bog standard Java.

However, some other projects around here using different application frameworks are stuck since the frameworks aren’t maintained or upgraded in ways that aren’t compatible anymore.

Looking into old Java code, it is hard to remember a time before enums and what a pain that is to deal with int constants instead of typed constants.