I've just installed Windows 8 from an MSDN ISO, activated with a DreamSpark key, and updated. Now the store shows the free update to Windows 8.1, but as far as I know, it shouldn't be available for MSDN Win8 installs. Does anyone know whether I'm safe to try to run the 8.1 installer? Will it work with my activation code? I don't want to mess it up, it's a one-time activation.
There should be an ISO on MSDN that will allow you to update. I wouldn't advise updating using the store. See this for details.