Hmm... The first link from post #3 (ZDNet's Ed Bott) seemed to indicate that he had no trouble in January 2018 with using a key from a previous Windows version.

Windows 7 should be OK until early 2020 with security updates. If you "upgrade" to Windows 10 after that, you could use StarDock's software to restore a Windows 7 type user interface, assuming that Microsoft hasn't done something to prevent it by then and that Stardock has kept up with any updates needed.

The problem with Windows 10 is that there really isn't any such thing as "Windows 10". Microsoft continually sends out "feature updates" that make radical changes to the user interface and settings, so they are really up to "Windows 14" by now. The build number is vital information to know which "Windows 10" version you are using so you know how to perform any particular function. For example, "Control Panel" has been removed from the latest build (although there is a way to get it back), and the functions are now scattered around elsewhere.

I have only one Windows 10 test system at present, and it is running Enterprise so I can lock it down. I am going to try to set up an additional test system with Enterprise LTSB (Long Term Service Branch), which gets only security updates automatically. If I can figure out how to do this, I'll make some documentation on it.