Just to close the loop I was able to eventually get this to work, was pretty convoluted though. I had 8.1 Pro activated via KMS and had my legit WMC upgrade key. As stated earlier, could not add WMC to Windows through System as was KMS activated. I created a Windows 8.1 WMC install via hotfixdownloader but did not input my key. I made sure to edit ei.cfg to reflect Windows 8.1 WMC and when I clicked setup on the ISO in Windows 8.1 KMS activated environment was given the option to install and keep my files. Unfortunately this did not keep all of my files and I have had to boot into a recovery environment to restore them from their deleted state, but I was able to install Windows 8.1 WMC on top of a previously KMS activated Windows 8.1. Upon first boot it said it wasn't activated though and wouldn't activate with my key so I did phone activation and was eventually given a confirmation code and now have Windows 8.1 WMC activated and am in the process of reinstalling my programs and restoring the above mentioned deleted files. This was a big hassle and I would not recommend going from KMS activated to WMC version of 8.1. As other members have advised would be better to backup files and settings and just do a clean install. Was able to get it working finally though.