You Can Still Use an Old Key with the Anniversary Update
As part of Windows 10’s November update, Microsoft changed the Windows 10 installer disc to also accept Windows 7 or 8.1 keys. This allowed users to perform a clean install Windows 10 and enter a valid Windows 7, 8, or 8.1 key during installation. Windows 10 would then report that key to Microsoft’s servers, and Windows 10’s actiation servers would give your PC a “digital entitlement” (now a “digital license”) to continue using Windows 10 for free, just as if you had upgraded.
This also works from within Windows 10. Even if you don’t provide a key during the installation process, you can head to Settings > Update & Security > Activation and enter a Windows 7 or 8.1 key here instead of a Windows 10 key. Your PC will receive a digital entitlement.
Now, even though the free upgrade offer is technically over, this method still works in the Anniversary Update, either when installing Windows 10 with Anniversary Update media or by entering the key after installing Windows 10. Enter any Windows 7, 8, or 8.1 key that hasn’t previously been used to upgrade to 10, and Microsoft’s servers will give your PC’s hardware a new digital license that will allow you to continue using Windows 10 indefinitely on that PC.
Microsoft hasn’t released any sort of statement about this upgrade method at all. It’s possible that Microsoft will disable it soon, but it’s also possible Microsoft will look the other way and keep this trick around to encourage more Windows 10 upgrades for a long time to come. Microsoft isn’t talking, so we have no clue.
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