PRODUCT KEY AND WINDOWS 10 ACTIVATION FROM WINDOWS 7/8
You can activate the clean install on the changed hardware , if you have your old Windows 7 or window 8 product key, and you have also updated your copy of (installed)Windows 10 to the November 2015 Update or later.
If you try to upgrade from Windows version 7 or 8 PC to Windows version 10, you won’t be able to get a unique product key. As your license will be associated to the hardware you upgraded (Windows 10) on, not your Microsoft account. Moreover, Clean installs will automatically activate as long as you do them on the same hardware.
The difficulty comes here because these keys are not tied to any hardware making it impossible to find. But here is a good news too after the “Windows 10 Anniversary Update and over (including the latest Fall Creators Update)”, you can now link these retail and digital licenses of Windows 10 to your Microsoft account.