Tested, tried and TRUE! I bought a Windows 8 Pro Upgrade a couple of systems back but never ended up using it. Now, I built a brand new system that I want to get Windows 10 on so I needed to use this upgrade.
If you’ve already formatted your drive or you’re moving to a new hard drive, you are not able to perform a “clean install” without installing an older version of Windows first. You are allowed to install Windows 8 cleanly, but when you go to activate, you get an error 0x8007007B saying your product key can only be used for upgrading.
If you get that error, here’s how to fix it:
1. Press the Windows key and type regedit
2. Press enter to open the Registry Editor
3. Navigate to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/ and double-click on the MediabootInstall key in the right pane
4. Change the key’s value from 1 to 0
5. Exit the Registry Editor
6. Press the Windows key again, and type cmd
7. Right-click on the Command Prompt icon and run it as an administrator
8. Type slmgr /rearm and press Enter
9. Reboot Windows
When you get back into Windows, you should be able Activate Windows as normal without getting an error. You could use this trick for evil but it is not recommended or encouraged.