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.
It took me a few hours to sort this all out on my own on Friday, but now that something like an "official" guide for Ruby developers has been released, I'm glad to find that I ended up choosing the preferred path!
After recompiling Ruby 1.9 as a 64-bit binary, installing the 64-bit version of MySQL, and getting the rest of my gems back in shape, things worked quite well.
That said, I'm not looking forward to spending several days helping people out with upgrades at work, haha. This one was quite a bit more involved than I expected.
NB: Had to do the same for Python's MySQL bindings with MySQL-64.
2) Bash is not running scripts that used to run (and it's not just a colour issue)
3) Macports broke and won't update itself, I've installed 1.8 over the top, but don't like fixing issues like this (especially apps that are critical to me)
Just looking at getting Ruby working now. I wouldn't advise anyone updating a work machine yet until more people have worked through all the issues.
But you should never be using the system Ruby anyways.