Bankesy: That is it. If you don't know your serial of Win 7 and you already did a clean install of Win 10 you can recover the key from the BIOS with this command
I got this from a Microsoft Employee after I did a clean install of Windows 10 on a new HP laptop without checking the serial first. So I couldn't activate Win 10 because I didn't have the serial and I couldn't reinstall Win7 because I didn't have the serial. So I called Microsoft and they explained that the key is stored in the Bios and that you can just find it by entering the command. If you then enter it as a serial in Win 10 it will activate without any issues.
