Is it possible to use the Windows 8.1 RTM Build from MSDN with an existing 8.0 Key?
It is indeed possible but you have to use one of the generic Windows 8.1 license keys below to install Windows 8.1 ( i.e. upgrade Windows 8.0 with the installation media ). The Windows 8.1 installer from the ISO does not accept Windows 8 keys. You need to install the system with a generic key and then enter your Windows 8 key after installing.
Note to moderators: these are not usable Windows keys. They are embedded in Windows itself as a placeholder default key and do not let you activate.
If you install the update through the Windows Store this isn't required. If you have
Windows 8 Professional with Media Center you can also skip the step of having to basically downgrade to
Windows 8 Professional then back to
Windows 8.1 Professional With Media Center.
In order to do this I was force to use a generic license key which are stored on the Windows 8.1 installation media. Once the installation was done I was able to use slmgr.vbs to change my product key to a Windows 8.0 Upgrade license. I was running Windows 8 Professional so I did that by running the following two commands.
slmgr -ipk XHQ8N-C3MCJ-RQXB6-WCHYG-C9WKB
Once you have performed this command you should restart your computer. Once the machine has been restarted you can attempt to activate Windows and when prompted provide your Windows 8 license key.
The quoted text above assumes you installed Windows 8.1 using a MSDN Windows 8.1 key, but the process still works, even if you installed Windows 8.1 with the generic key. All you do is remove the key using
slmgr -cpky, restart the machine, and attempt to activate the installation and when prompted provide your actual Windows 8 license key.