You won't have to redo any of your IS/ISBoxer setup.

Upgrading to Windows 7 should be pretty painless as far as multiboxing goes. If you're using Windows XP, then the worst case is you will now have UAC and will have to run some programs as Administrator (same as if you were upgrading to Vista). If you're using Windows Vista, then everything should operate basically the same (but perform better). It should be just that simple, even if you're going from 32-bit to 64-bit, etc. Just upgrade Windows and it should still work as it did before