When Microsoft unveiled Windows 10, it made the shocking announcement that upgrading from Windows 7 and 8.1 would be free for the first year. Computer users used to paying $99 or $129 for Windows upgrades were overjoyed.
However, the free upgrade process caused some problems. In the past, you could do a "clean install" upgrade of Windows using the product key of your previous version. With Windows 10, you would have to do an in-place upgrade, format the hard drive, then install a clean version of Windows 10. There was also the question of how you would re-install it in the future on new hardware.
Microsoft promised to fix the confusion and that fix arrived in the recent Windows 10 Fall Update. Now, as long as you have a valid product key for Windows 7 or Windows 8.1, you can activate Windows 10 for a clean install or future re-install.
To register your Windows 7 or 8.1 product key in a current installation of Windows 10, go to Settings>>Update & Security, select Activation and click the "Change product key" button. Now Windows 10 will be associated with that key.
Just as an aside, when you're on the activation screen, you might notice it says, "Windows 10 on this device is activated with a digital entitlement." This means that Windows 10 upgraded from a valid installation of 7 or 8.1, and is tied to your hardware.
That means re-installing Windows 10 on your computer as-is would be easy, but trying it after a hardware upgrade, or moving it to a new computer, would have been a problem. Hopefully, that problem should now be in the past.