I installed a fresh instance of gitosis, and also have an old one.
What do I have to do to move the repos from the old to the new server? Clone on the new server?
What do I have to change on the developer computers, to switch the remote branch? Or do they need to clone again?
Just change the address of the remote "origin" to the new location in your local checkout and push. You can change the address using git remote or you can edit .git/config directly.