Upgrading Windows 8.1 to 8.1 Pro fails with "This key won't work. Check it and try again, or try a different key"

I can't seem to find a way to upgrade my laptop Windows 8.1 (64 bits) OS to Windows 8.1 Pro. I do have a key for the new version but when I input it in the Windows updater, it tells me:

This key won't work. Check it and try again, or try a different key.

I'm using my MSDN and I know nobody used those keys.

To enter the key, I go to Control Panel - System, I see Windows Edition as 8.1. I click on "Get more features with a new edition of Windows" link, and in the popup "Add features to Windows 8.1" I select "I already have a product key".

Should I maybe just download the entire Wind 8.1 Pro ISO and run the install instead?
I would imagine the Windows Upgrade Tool would do it in a cleaner way.

Answer

For most of my clients, it wasn't working because by default the locale for everyone in MSDN downloads is UK, no matter where you live. If you search "Windows 8.1" by default the first result that shows up is the UK Edition, If your locale is the U.S., make sure to search for "Windows 8.1 (multiple editions) (English)" in your MSDN product keys.

Note: If you're running Windows 8, you must use a Windows 8 Pro product key and not a Windows 8.1 Pro product key. If you are running Windows 8.1, then you must use a Windows 8.1 Pro product key.