Just don't - really, dont. Fresh install. It took me ages and was a pain, but not as much a pain as the unsureness and niggles I encountered after a day on an upgraded Snow Leopard. It's so much better fresh.
At a low level, libraries tripping over each other. At a higher level, just weird interface "quirks" that would come and go. No such issues after a proper install - it's been entirely issue free.
