How to change the database name in oracle ?

SQL> select name from v$database;

NAME
---------
ORA9
SQL> alter system switch logfile;
SQL> alter database backup controlfile to trace;
The instance needs to be shut down. SYSDBA privileges will be needed.
SQL> shutdown
Modify (and optionally rename) the created trace file:
1. Find the line reading # Set #2. RESETLOGS case
2. Remove all lines above this line.
3. Change the line containing the database name fromCREATE CONTROLFILE REUSE DATABASE "ORA9" RESETLOGS NOARCHIVELOG
to
CREATE CONTROLFILE SET DATABASE "CAT" RESETLOGS NOARCHIVELOG
Note, in my case, the database is running in noarchive log mode. The corresponding line reads ARCHIVELOG otherwise.
4. Remove the line reading RECOVER DATABASE USING BACKUP CONTROLFILE.
5. Remove lines starting with #.
It looks then something like:
STARTUP NOMOUNT
CREATE CONTROLFILE SET DATABASE "CAT" RESETLOGS NOARCHIVELOG
-- SET STANDBY TO MAXIMIZE PERFORMANCE
MAXLOGFILES 16
MAXLOGMEMBERS 2
MAXDATAFILES 30
MAXINSTANCES 1
MAXLOGHISTORY 454
LOGFILE
GROUP 1 '/home/oracle/databases/cat/redo1.ora' SIZE 100M,
GROUP 2 '/home/oracle/databases/cat/redo2.ora' SIZE 100M,
GROUP 3 '/home/oracle/databases/cat/redo3.ora' SIZE 100M
-- STANDBY LOGFILE
DATAFILE
'/home/oracle/databases/cat/system.dbf',
'/home/oracle/databases/cat/undo.dbf',
'/home/oracle/databases/cat/data.dbf'
CHARACTER SET WE8ISO8859P1
;
ALTER DATABASE OPEN RESETLOGS;
ALTER TABLESPACE TEMP ADD TEMPFILE '/home/oracle/OraHome1/databases/ora9/temp.dbf'
SIZE 104857600 REUSE AUTOEXTEND OFF;
In my case, I renamed the file to /tmp/rename_db.sql.
Move the controlfiles away so that they can be re-created..
$ mv ctl_1.ora ctl_1.ora.moved
$ mv ctl_2.ora ctl_2.ora.moved
$ mv ctl_3.ora ctl_3.ora.moved
The database name must be entered (changed) in the initSID.ora:
initXXX.ora
db_name = CAT
sqlplus "/ as sysdba"
SQL> @/tmp/rename_db
SQL> select name from v$database;

NAME
---------
CAT