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 the generic Windows 8.1 key 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.
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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.[Screenshot TBA]
There have been mixed reports that using a non-generic Windows 8.1 license key in order to install Windows 8.1 will prevent you from using an Windows 8.0 Upgrade license key. Its not clear based on these reports if its possible to get around this state. These same reports indicate that changing a MSDN Windows 8.1 key to a Windows 8.0 MSDN key was possible.
I suggest waiting until the upgrade can be installed through Windows Store if this is a critical system. Its clear based on the number of steps required to do this that, using a Windows 8.1 installation media, is not the ideal way to upgrade to Windows 8.1.
I suspect this problem comes from the fact all Windows 8.1 license keys are "full" license keys and not upgrade license keys.