def change_branch(self, branch_id=None):
<EOL>raise Unimplemented()<EOL>
Changes the service branch.

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