I've finally upgraded from a really old version. Namely v6.1.1.
While it was fairly painless, the upgrades all worked and didn't require anything special, it did take me almost 6 days to do it.
The only problem I had was the threads importer going from 6.5.5 that didn't quite work properly, but it got fixed very fast.
I can't say enough how much better the forum has got. v7 and onwards is just amazingly good. There's still some quirks but I'm working through them.
I've stuck with UBB.threads ever since WWWThreads v2.7. And though I've tried almost all big forums out there, I've never found anything close to threads. The support couldn't be better, the features are perfect and the code is clear and easy to follow (although it could use some refactoring and added APIs). As I've done quite a few importers and exporters between different systems I an say that no-one has nearly as solid a structure as Threads.
I just realized that I had an extra license as well, and now I'm thinking about what I'm going to do with it... It needs to be worthwile since I still have to pay for the member area upgrade.