Using the Recovery Catalog - RMAN Recipes for Oracle Database 12c - page 171

You'll receive an error stating that the recovery catalog version in the recovery catalog database is too old.
To be able to connect to the older recovery catalog, you must upgrade the recovery catalog in the following manner
(you'll have to issue this command twice, as shown in the example, by using the upgrade catalog command):
RMAN> upgrade catalog;
recovery catalog owner is RMAN
enter UPGRADE CATALOG command again to confirm catalog upgrade
RMAN> upgrade catalog;
recovery catalog upgraded to version 12.01.00.00
DBMS_RCVMAN package upgraded to version 12.01.00.00
DBMS_RCVCAT package upgraded to version 12.01.00.00
RMAN>
After the catalog is successfully upgraded, confirm the version of the recovery catalog in SQL*Plus, again logging
in as the recovery catalog owner, as shown in this example.
SQL> conn rman/rman
Connected.
SQL> select * from rcver;
VERSION
------------
12.01.00.00
SQL>
You've successfully upgraded your 10.2 version of your recovery catalog schema to the 12.1 release version.
How It Works
Not all catalog schema versions are usable with all target database releases. Please check the compatibility matrix
provided by Oracle to learn about which recovery catalog schema versions are compatible with a particular version
of RMAN. You can also refer to Recipe 6-12, which deals with the resolution of RMAN compatibility issues.
The RMAN client you're using can't be a more recent version than the target or auxiliary database to which you're
connecting. The recovery catalog schema version must be at least the same as the RMAN client version or greater.
You can't upgrade a virtual private catalog with the upgrade catalog command—you must upgrade the base
recovery catalog. When RMAN connects to the virtual private catalog the next time, it automatically performs any
necessary changes in the virtual private catalog.