You can buy an upgrade copy of Windows 7 (which contains both 32- and 64-bit versions) and install the 64-bit version. From a licensing perspective an "upgrade" from a 32-bit to 64-bit system is valid and you will be able to activate your 64-bit Windows 7 system.
BUT - you cannot do an "upgrade in place" - you'll have to reinstall the OS from scratch, migrate your accounts and reinstall the programs that you use.