Changing DBID

What is the DBID?
Is a unique identification number when the database is created.
DBID means database identifier in Oracle.The database identifier is a unique identifier. This is found in all datafile headers. The database identifier is used to identify the database a file belongs to. The database identifier is also found in the control files. It can therefore be gotten without access to the datafiles.The database identifier is important when the spfiles or controlfiles must be recovered. 

Why we want change DBID?
Imagin we have a database clon without use RMAN, for example cold backup, u cant not register this database in same recover catalog cause same DBID, RMAN take care of DBID, and will prompt next error:
RMAN-20002: target database already registered in recovery catalog
WARNING: This steps are very dangerous, after this step, backup database, this operation obsolete any previus backup and archive log files, cause u will have new database incarnation.
STEPS:
1- check DBID
SQL> select dbid,name,open_mode,activation#,created from v$database;
2- Put in mount state
SQL> shutdown imemdiate
SQL> startup mount
3- proced change
$ nid TARGET=/
4- New incarnation
SQL> shutdown immediate;
SQL> startup mount;
SQL> alter database open resetlogs;
SQL> select dbid,name,open_mode,activation#,created from v$database;
5- New password file:
$orapwd file=orapw$ORACLE_SID entries=12 password=SYS_PASSWORD
6- Backup datbase
Run backup cause after this steps all backups become obsolete.
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