After you perform the basic downgrade steps, some changes from the 15.5 upgrade still persist.
sp_audit "exec_procedure", "all", "sp_addlogin", "on"
isql -Usa -P <sa_password> -S <server_name> -i <restore_msgs.ebf>
If you upgraded from a 15.0 or 15.0.1 release and are reverting back to the 15.0 or 15.0.1 release, then you must only run instmsgs.ebf file from the relevant release.