You probably don't need to bother with 'reserving' your copy of W10 via the 'Get Windows 10' app. I've been told a fresh, unpatched install of W7 SP1 will upgrade just fine if you use an ISO.
I had already performed the Windows 10 upgrade once on the computer, so it was registered. I would guess if you're doing it for the first time you may have to activate the copy of Win 7/8 first, but maybe just having a valid product key is enough?