Change db_unique_name in RMAN catalog (needed if db_unique_name of the instance has changed)

First, you need to change the unique instance name of the database.

SQL> show parameter db_unique_name

$ rman target / catalog rman/rman123@rman12

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RMAN> LIST DB_UNIQUE_NAME OF DATABASE;


List of Databases
DB Key  DB Name  DB ID            Database Role    Db_unique_name
------- ------- ----------------- ---------------  ------------------
1       ORCL12   3487575625       PRIMARY          ORCL12

// If you need to change the unique database name in the rman catalog.

RMAN> CHANGE DB_UNIQUE_NAME FROM ORCL12 TO ORCL12C