With the release of Team Foundation Server 2015, it’s time to upgrade our existing TFS 2013 environments. One warning before you start, upgrading to TFS 2015 can take a long time(depending on the size of your database), a lot longer than upgrading previous versions. The reason for this is that TFS 2015 includes schema changes to a large number of tables in order to support renaming team projects.
If you want to minimize downtime, it is recommended to use TfsPreUpgrade.exe, a small tool that does the most expensive parts of the work to upgrade to TFS 2015 while your TFS 2013 deployment is still online. TfsPreUpgrade.exe works by creating new copies of the tables most impacted by the schema changes present in TFS 2015 and then migrating data from the original tables to the new copies. As TfsPreUpgrade.exe adds some triggers behind the scenes, it can have a negative impact on performance. So only run it when you will upgrade TFS in the near future.
Remark: TfsPreUpgrade.exe can provide a rough estimate of how long it will take to execute by running the Estimate command.
More information: https://msdn.microsoft.com/en-us/library/vs/alm/tfs/upgrade/pre-upgrade