I upgraded my wife's PC from 7 Ultimate to 10 Pro skipping the Product Key input screens. The 10 Pro activated.

I then clean installed 10 Pro skipping the Product Key input screens and it activated. Interesting fact, the clean install didn't show the roundy thingy progress screen. It showed the Windows 8.1 screens. This was an x32 clean install.

I had none of the issues shown on the forum. Luck of the Irish maybe.