Can you use old Win 7 product code on new computer after updating to Win 10?

[Windows Central Question]

Has anyone tried installing Win 7 on a new PC after updating the previous PC to Win 10 with success? Second question would be if you could then update that PC to Win 10 as well? Not trying to get endless Win 10's, I just plan on doing a new PC build this winter and was wondering if I should wait to Win 10 my current Win 7 until I put the Win 7 product key on the future PC.
Thanks