I'd say the best way to do it, is to A: upgrade 7 to 10 now. B: Do a search and download the free Key Finder from MagicJellyBean and write down the new Windows 10 product key on a piece of paper and save it. Also, copy the key into a notepad document and save it to OneDrive. C: Go back to Windows 7 then and install Windows 10 using your Windows 10 product key and do a clean install of it.
P.S. I know from experience, Windows 10 gives you a new product key. It doesn't give you the same product key as Windows 7 or 8.1.
You can only use the Windows 7 or 8.1 product keys to upgrade to Windows 10 until July 21, 2016. After then, you need to pay for it.