The story started with finding a “good” seller for Windows 10 Pro Keys. There are currently a lot of fake/used Windows keys in Ebay and Amazon. So I bought one from a reputable reseller in Germany/Austria. The only thing that did not seam quit right was the label “Windows 10 Pro DVD OEM” under which they are selling the item. As far as I know, Windows 10 OEM keys should only be used by system integrators for example for a laptop or computer that you buy at a store with Windows being pre installed. I bought the key anyway because OEM keys are usually up to 60% cheaper than retail keys.
The problems started, when I was trying to change the key in the Windows Aktiviation Dialog. A yellow message was displayed with the Error “0xc004f050” after pressing enter. I tried this with the original install from the OEM and a fresh install on the computer. It seams, that upgrading with an OEM key is not possible. It was also not possible to choose the edition of Windows when I tried to install Windows 10 Pro from the USB-Stick, since the Windows 10 Home key was stored in the UEFI.
The thing that eventaully worked for me is to add a file to the Windows Boot Media (in my case a USB-Stick). In the sources folder in the device add a file with the name ei.cfg and the following conntent. [EditionID] Professional [Channel] Retail
This forces the install programm to install Windows 10 Pro, or at least display you the option of editions when installing Windows 10 on your computer.
After one reboot an the inital setup windows showed the default Windows 10 Pro key and displayed that Windows could not be registered.