I didn't even have to do this for 8.1, I got the $40 upgrade way back when, and thats the key I used. Upon clean-installing 8.1 I activated with that upgrade key and it didn't complain at all, just activated. Which is odd, because with windows 7 I always had to use that workaround, and I recall doing it when windows 8 first came out as well (the mediabootinstall registry hack).
I did notice when I checked regedit on my win8.1 machine mediabootinstall was already set to 1, I opened up regedit after installing all ready to make the change before trying to activate and it already was, so I'm not quite sure why that is or if thats been the same case for anyone else, or if that registry key even matters anymore.
Ok, thanks for the info.