I'm not gonna use the license simultaneously on both PCs. I'll erase it on one pc and install it on another.

Question 1:
I own a windows 7 retail disc copy which i can use to reinstall and activate on a fresh new pc,migrating my license to that new pc. But if I free upgrade to windows 10 on my old PC...

I have a system builder version of Windows 7 installed on my old PC which I can upgrade to Windows 10 when available. Suppose if I build a new PC after one year when no more free upgrade will be...