All Windows 7 product keys are valid for both 32-bit and 64-bit (excluding Win 7 Starter). It's up to you which architecture to install.
The only difference between buying a 32-bit version and a 64-bit version is the installation media that comes with it.
Unfortunately the only way to switch is with a full re-install of Windows. If you want to go ahead with this you can download the 64-bit version of Windows from links on this site - http://www.w7forums.com/threads/official-windows-7-sp1-iso-image-downloads.12325/
The downloads are 100% legitimate and come from Digital River, an official Microsoft Partner