I hate to say it, and I can garauntee that someone will post after me saying it can be done perfectly safely and everything but personally something as major as a mobo change will require a rebuild of the OS. There are just WAY to many things that change, and running ME, I wouldn't really give it a hope of staying stable (Using the word 'stable' lightly of course!).
I honestly think that backing up all data and starting from scratch is going to be FAR less troublesome than trying a motherboard change with a live OS.
Just my opinion however.