We are maintaining code for one of our clients. Initially, we copied all the source code that they have and added it to our TFS 2012. We modify the code any time they need a bug fix and give the client deployment packages.
Now, client wants all the latest code in their TFS 2012 as well.
Is there a way to update their source code with our changes? ... preferably automatically (i.e. power shell script) and preferably with history of changes.
There are many approaches each with some pros and cons. The following are the main options I would suggest.
This is the only path that guarantees full fidelity. It has some technical difficulties (e.g. SQL Server version and editions) and political (how much information you care to expose, how much effort you want to put in sanitizing your data).
There are some tools, most notably the Integration Platform, that use the API to read and reply the changes from one system to the other. It requires that the syncing tool can see both systems via HTTP(S). It gives you the flexibility to project only some data (say source code not work items). Keep in mind that you will always loose something in the process: the Changeset number will never match, some users details.
Give up conserving full history and be content to share the code. This is the simplest to implement: get all the code, ship and check into the other system. You can associate release notes in the check-in. Two simple scripts using TF.exe is all you need.
You can use TFS Integration Tool to achieve the code migration(TFS-to-TFS). TFS Integration Tool moving data between two different servers. The migration is done through the APIs of TFS, and there also some limitations.(Check the above link for more info)
Detail steps please see my answer in this question: Move Team Project to another Project Collection TFS 2013
