Database characterset migration

In most cases a full export and import is recommended to properly convert all data to a new character set. However if, and only if, the new character set is a strict superset of the current character set, you can use the ALTER DATABASE CHARACTER SET statement to expedite migration to a new database character set. The target character set is a strict superset if each and every code point in the source character set maps to the same corresponding code point value in the target character set.

Also read
http://www.oracle.com/technology/tech/globalization/pdf/mwp.pdf