Thank you for the reply. But this confuses me because I have used my Windows 8.1 Pro key on other computers (that never had any version of Windows installed on them previously) to activate a clean install of Windows 10 pro without having to install Windows 7 first? *Edit: I do have my own Windows 8.1 product key. Just to clarify, this is an upgrade from Windows 10 Home to Windows 10 Pro. Thanks!
You can activate Win 10 Pro if you have a old Win 7,8, or buy a Win 10 key, the default one will not work, if you have old one or buy new one you can enter it in what you have now.
The default key was only used to initiate the upgrade process from Windows 10 Home edition to Windows 10 Pro edition. I have an old Windows 8.1 key that I was trying to activate after entering the default key. As it turns out, the reason my old key was not working was because I needed to update to version 1511 of Windows 10 first. After running windows update, it accepted my old Windows 8 key just fine.
I had a similar problem trying to upgrade. I tried phone activation and that didn't work either but after rebooting and entering a valid 8.1 pro key it activated fine.