Try reading the Oracle8i National Language Support Guide document... it should guide you properly.
Here's an excerpt that is pertinent:
In most cases, you will need to do a full export/import to properly convert all data to the new character set. However, if, and only if, the new character set is a strict superset of the current character set, it is possible to use the ALTER DATABASE CHARACTER SET statement to expedite the change in the database character set.
The target character set is a strict superset if and only if each and every codepoint in the source character set is available in the target character set, with the same corresponding codepoint value. For instance, the following migration scenarios can take advantage of the ALTER DATABASE CHARACTER SET statement because US7ASCII is a strict subset of WE8ISO8859P1, ZHS16GBK, and UTF8:
Table 3-8 Sample Migration Scenarios Current Character Set New Character Set New Character Set is Strict Superset? US7ASCII WE8ISO8859P1 Yes US7ASCII ZHS16GBK Yes US7ASCII UTF8 YesAttempting to change the database character set to a character set that is not a strict superset can result in data loss and data corruption. To ensure data integrity, whenever migrating to a new character set that is not a strict superset, you must use export/import. It is essential to do a full backup of the database before using the ALTER DATABASE [NATIONAL] CHARACTER SET statement, since the command cannot be rolled back. The syntax is:
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 them are absolutely necessary, but they are highly recommended:
SQL> SHUTDOWN IMMEDIATE; -- or NORMAL <do a full backup> SQL> STARTUP MOUNT; SQL> ALTER SYSTEM ENABLE RESTRICTED SESSION; SQL> ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0; SQL> ALTER DATABASE OPEN; SQL> ALTER DATABASE CHARACTER SET <new_character_set_name>; SQL> SHUTDOWN IMMEDIATE; -- or NORMAL SQL> STARTUP;To change the national character set, replace the ALTER DATABASE CHARACTER SET statement with the ALTER DATABASE NATIONAL CHARACTER SET statement. You can issue both statements together if desired.
This was first published in December 2002