When I was dabbling with Hackintoshes a few years ago, updating the OS was not supported (regardless of your hardware) and there were a couple of other limitations. Have most major issues like that been sorted out?
Which I did/do for my Macbook Pro anyway. So when I got into the Hackintosh scene, it's second nature. I typically recommend formatting when upgrading anyway. Just doesn't feel right to just upgrade. Maybe it's just left over nightmares from when I was on Windows. Only way to upgrade was to format.