ORA-12712: new character set must be a superset of old character set.

We can get the following error while changing the character set in oracle.
SQL> Alter database CHARACTER SET UTF8;
Alter database CHARACTER SET UTF8
*
ERROR at line 1:
ORA-12712: new character set must be a superset of old character set

Then what we have to do,
SQL> ALTER DATABASE CHARACTER SET INTERNAL_USE UTF8;

Database altered.

SQL>
SQL> SHUTDOWN IMMEDIATE;
Database closed.
Database dismounted.
ORACLE instance shut down.

After restarting verify the nls_parameters..
SQL> select * from v$nls_parameters;