I've never purchased Windows 10 but have it on multiple computers.
I have a few systems that used to have Win 8.1 on them that were upgraded to Win 10 when it first came out.
I have a couple of systems that I built but instead of buying Win 10, I downloaded it from MS using the Media Creation Tool. Then, during the clean install of Windows 10, I provided an old Win 7 license when prompted. Worked like a charm.