Oracle8i SQL Reference
ALTER DATABASE CHARACTER SET ...

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CHARACTER SET

NATIONAL CHARACTER SET
CHARACTER SET changes the character set the database uses to store data. NATIONAL CHARACTER SET changes the national character set used to store data in columns specifically defined as NCHAR, NCLOB, or NVARCHAR2. Specify character_set without quotation marks.


WARNING: You cannot roll back an ALTER DATABASE CHARACTER SET or ALTER DATABASE NATIONAL CHARACTER SET statement. Therefore, you should perform a full backup before issuing either of these statements.


Restrictions:

You must have SYSDBA system privilege, and you must start up the database in restricted mode (for example, with the SQL*Plus STARTUP RESTRICT command).

The current character set must be a strict subset of the character set to which you change. That is, each character represented by a codepoint value in the source character set must be represented by the same codepoint value in the target character set. For a list of valid character sets, see Oracle8i National Language Support Guide.
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Сам никогда не пробовал. Export/пересоздание/import роднее как-то :-)