sqlplus /nolog

connect sys as sysdba

--------------------
-- these parameters have to be changed
-- in order to change characterset
--------------------
Alter system set job_queue_processes=0 scope=spfile;
Alter system set aq_tm_processes=0 scope=spfile;

shutdown immediate
startup nomount
alter database mount exclusive;
alter database open;
alter system enable restricted session;
purge dba_recyclebin;
alter database character set internal_use AL32UTF8;
alter database national character set internal_use UTF8;

shutdown immediate
startup