def _rename_gtfs_zip(gtfs_dir, old_filename, gtfs_name, modify_date):
    _create_dir(gtfs_dir)
    new_filename = os.path.join(gtfs_dir, '{}_{}.zip'.format(gtfs_name, modify_date))
    if os.path.isfile(new_filename):
        if _compare_files(old_filename, new_filename):
            _progress('downloaded gtfs file is identical to: {}'.format(new_filename))
            os.remove(old_filename)
            return None
    _rename_existing_file(new_filename)
    os.rename(old_filename, new_filename)
    _progress('renamed: {} -> {}'.format(old_filename, new_filename))
    return new_filename