I am going to to upgrade to windows 10. I want to first upgrade my Win 8.1 home to 8.1 pro. I'd rather not pay the $100 Microsoft wants, plus since Win 10 has been out if I go to the System in the Control Panel and select "Get more features with a new edition of Windows" it no longe gives me the ability to purchase only to enter a key I would like to use a global cd key or if possible oem key (it's cheaper) for Windows 8.1 pro. Are either of those an option? If so, which ones? Where do i enter the key? Is it the add features to windows 8.1 or what?