def change_branch(self, branch_id=None):
Changes the service branch.

        arg:    branch_id (osid.id.Id): the new service branch
        raise:  NotFound - ``branch_id`` not found
        raise:  NullArgument - ``branch_id`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  PermissionDenied - authorization failure occurred
        raise:  Unimplemented - ``supports_journal_branching()`` is
                ``false``
        *compliance: mandatory -- This method must be implemented.*