Recently I have done a TFS upgrade from 2012.2 to 2013.4 with a hardware move. DNS settings changed to access new TFS server using the previous TFS URL. All went well and I when I started TFS 2012.2 build farm (plan to upgrade them later) it was working as expected.
Started the test controllers and when viewed in MTM Lab Center the controllers shown offline.
My plan was to upgrade the test controllers later since currently lot of lab environments in use. Event viewer in test controller indicated an error.
Restarted test controller service, restarted test controller machine, but was not helpful.
Tried clean TFS cache in Test Controller machine.
All of above was not helpful. So I decided to take a risk and reconfigure the test controller 2012.2 with the new TFS (2013.4) server. My only worry was what will happen to the existing lab environments after reconfiguring test controller.
After reconfiguring everything was smoothly working. All my previously up and running labs were working as expected.