I have Windows 7 Professional (64Bit) installed and attempted to upgrade to Windows 7 Ultimate (64Bit). My DVD has 32Bit and 64Bit. Clicking on 64Bit and going forward, I receivee a message saying that I can't upgrade to 32Bit from 64Bit and the installation (upgrade) stops. As stated, I clicked on 64Bit as previous edition.
By changing the shown Registry keys to Ultimate exactly as shown, it tricks the installer into thinking you are doing a Repair Install (same version Upgrade over itself) which is allowed.