Rman catalog can be upgraded to a higher version than its hosting database oracle version. This is useful if you want to back up new versions of oracle without changing the oracle version on the database used to host the rman catalog.
To do this, run the following commands from the higher version oracle database:
rman connect target / #< can be a higher version of oracle connect catalog username/password@connect_string #< can be a lower version of oracle upgrade catalog ;
Tested on v18.104.22.168 - v22.214.171.124 on linux RHEL5 64 bit.