It seems to be all over the place! Some keys work, some don't...what i found was retail 7 WOULD activate 10, manufacturer OEM would USUALLY activate 10, online purchased OEM would NOT activate 10.
not that i know for 100%, i probably only tried with 40 or 50 computers.
i only ever had a couple windows 8 keys to try and they activated 10 fine. they were manufacturer OEM ON THE ORIGINAL DEVICE.