Sorry, but you can only do a clean install when going from 32-bit to 64-bit, or the other way around.
The main thing is to make sure that you have 64-bit drivers
available for all of your devices, and that your 32-bit programs will support and run in 64-bit Vista. Most do, but some utility or system type programs may not.
This link below can help explain the main differences between a 32-bit and 64-bit OS though.
64-bit: More than just the RAM - Vista Forums
Hope this helps,
This was copied from a Brink's post to a similar question.