You can't upgrade 32 to 64 - you'll need to reformat and reinstall. If you download the Win7 64bit ISO, your license key for the 32bit version will work (the key is for Premium, any number of bits).
XP to 7 works, yes (although that wasn't an upgrade - while the upgrade disk works, it does a clean install from XP to 7). But this is Win7 32 to Win7 64, where you can't do an in-place upgrade.