Not disputing that, Just saying that you can activate with your upgrade key. Installing WMC is completely optional and independent of activating your upgrade key (39.99 or 14.99 keys). I was afraid that the upgrade key was going to be useless for doing an upgrade install, it appears that is not the case.
All threads here regarding Win 8.1 install hint on that fact in OP . It was posted countless times here so far .