Someone sent me this link about the new 64-bit Windows XP. There's some interesting text on that page I would like to quote here (assuming the link doesn't change...):
I can tell you with confidence that if you go from Solaris 9 on x86 to Solaris 10 with both x86 and x64, you will NOT have to erase your hard drive (assuming you've enough room for the new x64-native binaries, and you probably do already if you're in possession of an x64-capable box).