Local Accounts - Upgrade Question

I am building a new tower for my in-laws and nephews. As of now, they want to stay with Windows 7. I'm fine with that, and have a licenses for them. However, I would like to upgrade it to Windows 10 temporarily, to get their license activated. I will roll it back to Windows 7 and use another key for the time being.

Now here's the question. Normally, I could do a clean install of Windows 10 later on. However, non of them have a Microsoft Account. How would the license be tied to their computer in the future? I recently upgraded my laptop to Windows 10, then was able to do a clean install. Once I logged in using my Microsoft Account, my laptop was activated. How would this be accomplished on a computer with only local accounts?

Is my only option to create a Microsoft Account just for this computer, so a Windows 10 license gets applied?

Thanks for the info. I was thinking it tied the hardware hash to the Microsoft Account. Good to know for future family and friend builds.