Keep in mind, most likely these will all be linked to the same Win7/8 key in a db somewhere, and as soon as they go online they'll likely all be deactivated, except for the last one This is how it worked wit Win7/8 for me, as soon as I activate a new machine, the old one is deactivated one it's online But, technically you can always find ways to circumvent licensing That's not the point here
If there are any links between the 7/8 retail keys and the upgrade Win 10 keys, then safest bet is still the OEM method. thanks for sharing your experience.