How to handle "The path X is already mapped in workspace Y"

Take a look at http://blogs.msdn.com/b/buckh/archive/2006/09/12/path-is-already-mapped-in-workspace.aspx.

Shortly, you have to remove Team Foundation Server cache that store at "C:\Users\{User}\AppData\Local\Microsoft\Team Foundation\3.0\Cache" named "VersionControl.config". After, you remove it, Visual Studio won't remember the map path from your local path to source control server, then you can map that path to the new server freely.