NID

NID (also called NEWDBID) is a utility in the ORACLE_HOME/bin directory that can be used to assign a new DB ID (and optionally a new DB_NAME) to a database. Usage:

Ensure your database is in mount mode:

sqlplus "/ as sysdba"
startup mount
exit

Invoke the nid utility to only change the DBID:

nid target=/

Invoke the nid utility to change the DBID AND DB_NAME:

nid target=/ dbname=orcl2

To verify your DB ID and database name:

SELECT dbid, name FROM sys.v$_database;