Migration from Java 8 is very easy, I don't really know why they keep using old versions. Probably lack of qualification. Or project is just dead with pinned ancient dependencies of terrible libraries which rely on JVM internals. I'm using Java 8 with one project, because that project must run on Windows Vista and later javas don't run on it. As soon as Windows Vista would become irrelevant, I'll drop the support in one day.
The problem with Java migrations is sometimes people (ab)use the behavior of a specific library in that specific version, and when you change the runtime, everything comes crashing down (visibly or invisibly).
I have seen quite a few Java applications which fails at unrelated points, and every time I have been told that the thing only runs with "jre-old.very.specific.version-and.point.release". Otherwise you're out of luck.
