When you say "upgrade", do you mean you do the actual package updates or do you install the new OS from scratch? Doing the former between versions with such huge difference, while in theory should work, sounds like a pretty bad idea to me. If you're thinking about installing from scratch, just try it from the live disk first and see how it works for you.
Ok, that's what I thought. I really wouldn't recommend trying to upgrade further. Gnome 3 and Gnome 2 can not exist together, and it's hard to say in what state your system would end up. I'd backup everything of relevance, and install Mint 14 from scratch. Or not touch anything at all, depending on your reasons for upgrading.