The Team Foundation Server Integration Platform is available as a free platform for migrating/synchronizing data between TFS servers and with Visual Studio Online.
The ALM Rangers have an article in MSDN Magazine with a walkthrough: http://msdn.microsoft.com/en-us/magazine/jj130558.aspx
There are a couple of limitations from what I understand:
- Takes some work to get up and going but the walkthrough helps out a lot
- This tool does migrate test cases but you need to run the UpdateSharedSteps tool from the product team.
- Not actively invested in as new features in Team Foundation Server and Visual Studio Online lights up