I have Windows 7 running installed on my desktop on 1 hard drive. I have installed but not formatted a 120 Gb SS drive which will hold Windows 10. I have already downloaded the Win10 ISO using the Media Creation Tool, written it to a DVD and checked that it boots.
I plan to do an upgrade by booting from the ISO DVD, and let it "pick up" the product key from the Windows 7 installation.
EDITED: I installed Magical Jelly Bean Keyfinder and it shows me the real Product Key and Product ID, matching one of the two packages that I purchased. What SYSTEM shows is only the Product ID! The rest of my question is now not relevant so am deleting. The bit above about the Keyfinder utility may be useful to some people.