So you should have both x64 and x32 discs. Are you wanting to know if you can change it without reinstalling? If so, I don't think so, but I'm not 100% sure.
EDIT: Apparently not..
Can I upgrade from a 32-bit version of Windows to a 64-bit version of Windows?
No. If you are currently running a 32-bit version of Windows, you can only perform an upgrade to another 32-bit version of Windows. Similarly, if you are running a 64-bit version of Windows, you can only perform an upgrade to another 64-bit version of Windows.
Just get 7 and be done with it