For over a decade since the launch of Windows 8, Windows has had a not so well kept secret; you could activate an install using a Windows 7 key. So long as you had the correct version of the key for the install you intended to use, such as Home or Pro, plugging a Windows 7 key into a Windows 8, 8.1, 10 or even 11 install after upgrading would activate that new copy of Windows. Microsoft originally allowed this as a means to entice users away from Windows 7 and onto their newer operating systems via the free Windows Upgrade system. This even worked when doing a fresh clean install if you had an OEM key harvested from an old machine. That old Dell box in your closet that came with Windows 7 Pro in 2009? That key glued to the back could still be used to activate a fresh copy of Windows 10/11 in 2023. Same with Windows 8 keys. That changes soon.

Microsoft announced the change on their Device Partner Center in a very short posting which reads,

Microsoft's free upgrade offer for Windows 10 / 11 ended July 29, 2016. The installation path to obtain the Windows 7 / 8 free upgrade is now removed as well. Upgrades to Windows 11 from Windows 10 are still free.