I'm sure you know exactly what you're talking about... but I can't really imagine what major incompatibilities you're seeing while doing these upgrades. Java is amazingly backwards-compatible in my experience. I don't think I've ever had to change anything during an upgrade, in my entire career, except sometimes a couple of minor things to do with Unsafe. I'd love to hear more and some concrete examples if you're able to.

I have no experience in upgrading Java versions or the corresponding code changes required. I have witnessed runtime differences, related to performance and GC behavior, from Oracle 8 to OpenJDK 8 that were unexpected enough to warrant temporarily sticking with Oracle in exactly one case.

That is eons ago, in Java 8 there were indeed differences between Oracle JDK and OpenJDK, this in no longer true since Java 10, even less so in Java 16.

Now, Java like C or C++, enjoys multiple implementations, so even across OpenJDK, J9, Azul, PTC, Aicas,..., there will be differences.

And that's why you have tests...

Having migrated some respectably sized projects to OpenJDK in my experience it's more work to migrate a codebase to a new Java major version than from Oracle to OpenJDK.

Are you serious? I've seen several large legacy Java applications move to OpenJDK without a problem.