Hi,
There's no other way to downgrade JoomSEF but uninstalling it and installing the older version again.
You should be able to preserve all your URLs and configuration though. First of all make a backup of your whole website files and database! Before uninstallation, backup the /administrator/components/com_sef/configuration.php file. When you uninstall JoomSEF, all the tables in your database starting with jos_sef will have the _bak suffix added, so no data are lost. When you install the older version again, just restore the configuration.php file from the backup, and in your database delete all the tables starting with jos_sef that don't end with _bak, and then rename the ones with _bak so that you remove the _bak suffix.