The way I understand a Windows 10 upgrade is that if you upgrade your Windows 7 key becomes a Windows 10 key and is no longer a valid Windows 7 key. So if I understand it correctly (and I may not) you'll be able to use your current Windows 7 key after you upgrade hard drives/motherboards even if you do upgrade to Windows 10, but it will only work if you re-install Windows 10, it will not work if you try to re-install Windows 7 again and then upgrade to Windows 10 again.

My two cents.