alter Oracle 10g character

sqlplus /nolog
conn SYS/1234 as sysdba;
shutdown immediate;
startup mount;
ALTER SESSION SET SQL_TRACE=TRUE;
ALTER SYSTEM ENABLE RESTRICTED SESSION;
ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0;
ALTER SYSTEM SET AQ_TM_PROCESSES=0;
ALTER DATABASE OPEN;

alter database character set AL32UTF8;
(****have Error like ORA-12712,please read note below )

ALTER SESSION SET SQL_TRACE=FALSE;

shutdown immediate;

STARTUP;

select NLS_CHARACTERSET from nls_database_parameters;

-------------------------------------------
****Note:
ERROR at line 1:
ORA-12712: new character set must be a superset of old character set

ALTER DATABASE character set INTERNAL_USE AL32UTF8;