I've upgraded my vista home premium to ultimate no prob..
Been using it ever since.. I just pop in the ultimate cd, follow the onscreen guide and after 20min or so it's done..
I didn't have to reinstall anything..
If the software works in the home premium it should work in ultimate too..
I upgraded the home premium 32bit to ultimate 32bit, don't know ar if it would work from 32bit to 64bit though..