According to Oracle:

"The ALTER DATABASE [NATIONAL] CHARACTER SET statement does not change character codes. Thus, before changing a fixed width database character set to a varying width character set (like UTF8) in Oracle 8.1.5 or later, you first have to export all tables containing non-NULL CLOB columns, then truncate these tables, then change the database character set and, finally, import the tables back to the database. The import step will perform the required conversion."