From the blog "Windows 10 MVP Insider Blog"
It looks like Microsoft is starting to make some attempt at closing the Windows 10 Free Upgrade loopholes. The Media Creation Tool, which provides detailed options for upgrading, now prompts Windows 7 or Windows 8 systems for a product key. The Media Creation Tool takes into consideration, users, who might have downgraded a previously installed and activated Windows 10 installation; by giving them the option to reinstall Windows 10, exempting them from the product key requirements. The new Digit