Order of source repository

It is possible to have both TFVC and GIT in the same TFS project, and when you create a new project you set the default repository and the afterwards you can add the second one.
An important notice about this:
If you run XAML based builds, the build definitions can only use the default repository aka the one you selected when you created the TFS project.

So if you still use XAML, make sure you select the correct repository when you create the project

It is not possible to switch over. At least not officially yet….
UPDATE: MS support has a created a tool that can fix this