CHANGING THE DATABASE OR NATIONAL CHARACTER SET
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In Oracle7 it is possible to update the "SYS.PROPS$" table to change the 
database character set. The method is unsupported but it is externally 
published (see <Note:13856.1>). It is used by Oracle Installer to install 
seed databases.

This method is not legal in Oracle8 because character set information is 
stored in many other places in the Data Dictionary besides SYS.PROPS$, for 
example with each table column, PL/SQL argument, etc.

In Oracle8 there is another way of changing the database or national character 
set. The method uses two commands, which are documented in the Oracle8i 
National Language Support Guide:

ALTER DATABASE [<db_name>] CHARACTER SET <new_character_set> 
ALTER DATABASE [<db_name>] NATIONAL CHARACTER SET <new_NCHAR_character_set> 

The database name is optional. The character set name should be specified 
without quotes, for example: 

ALTER DATABASE CHARACTER SET WE8ISO8859P1

To change the database character set perform the following steps. Not all
of these steps are absolutely necessary, but they are highly recommended:

SVRMGR> SHUTDOWN IMMEDIATE; -- or NORMAL

<do a full database backup>

SVRMGR> STARTUP MOUNT; 
SVRMGR> ALTER SYSTEM ENABLE RESTRICTED SESSION; 
SVRMGR> ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0; 
SVRMGR> ALTER SYSTEM SET AQ_TM_PROCESSES=0; 
SVRMGR> ALTER DATABASE OPEN; 
SVRMGR> ALTER DATABASE CHARACTER SET <new_character_set>; 
SVRMGR> SHUTDOWN IMMEDIATE; -- OR NORMAL
SVRMGR> STARTUP;