If you were one of the lucky ones who was able to acquire a Windows 8 MCE key from the online store, before Microsoft decided to hold off until October 26th to release them, then you may have run into this situation. There isn't an obvious way to downgrade from Windows 8 MCE to another version of the operating system. The same isn't true if you were going from Pro to Core for instance. So after some experimentation I was able to come up with this solution which I tested and worked.
Assuming you have your Windows 8 RTM media handy, you will be able to perform this "trick" on your machine. These also require some advanced techniques involving the registry under an administrator account. So if you are not used to using the registry this may seem daunting at first. Another prerequisite is that you have another Windows 8 machine, either physical or virtual, that does NOT have the MCE additions installed. In my case I used Vmware and installed Pro to a virtual machine and performed the following steps:
If you go to these locations in your registry (windows key + r then regedit) on the "Windows 8 Pro" vm/machine:
A: HKLM\software\microsoft\windows NT\CurrentVersion
B: HKLM\Software\Wow6432Node\Microsoft\Windows NT\Current Version
Right click each "Current Version" in each, first in part A.. export this to a file. Then right click registry key "Current Version" B and export that to a file.
NOTE: It may not be necessary to export "B", only A.. though I haven't fully tested that theory.
Execute the Exported Keys on the Windows 8 MCE machine
Once these keys were exported, I copied them to my Windows 8 MCE machine. Here I just double clicked each reg file.
Run the Installation Media
Immediately after importing the two registry keys I then inserted the RTM Windows 8 CD.
Here I chose the upgrade, keeping files and settings option, which is basically an in-place upgrade. I used a PRO key for this step.
After the Upgrade / Drawbacks
Immediately after I checked the version info. The results were that I now had Windows 8 Pro without the MCE portion and it was activated.
The downside to using this method is that after the upgrade some things needed reinstalled, almost like upgrading from Windows 7 as I previously outlined. In my case I had to reinstall Hamachi and a few other apps, but besides this everything has worked smoothly since "downgrading" from MCE.
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