"Run as different user" Visual Studio, cannot clear cached TFS credentials

Go to control panel > User Accounts > Manage Your Credentials > select your Team foundation Server and choose remove/edit - viola! Next time you go into Team Explorer you will be prompted for a new set of credentials.

Ref: http://blogs.msdn.com/b/visualstudioalm/archive/2012/08/29/clearing-the-credentials-for-connecting-to-a-team-foundation-server.aspx

You can try clearing the cache manually here:
C:\Users[USERNAME]\AppData\Local\Microsoft\Team Foundation\5.0\Cache

You could also try using the /profile parameter and specifying the username's profile when you run it.

Runas /profile /user:[DOMAIN]\[administrator] "C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\devenv.exe"

http://technet.microsoft.com/en-us/library/bb490994.aspx