Technically yes, although depending on whether it's retail (purchased from a store or online) or OEM (shipped with the PC) will determine if you need to call Microsoft or not. However, in either case, yes you can.
thanks for the fast reply.
I just want to make sure, so I can register x64 retail version on a computer that had x32 retail version registered.(of course I would format my main hard drive and install x64 version
If you do decide at a later time to clean install from 32-bit to 64-bit, here's a lot of useful information that may prove useful in these tutorials linked below.