Oracle Message ORA-12712 - new character set must be a superset of old character set

ERROR	new character set must be a superset of old character set
CAUSE	When you ALTER DATABASE ... CHARACTER SET, the new character set must be a superset of the old character set. For example, WE8ISO8859P1 is not a superset of the WE8DEC.
ACTION	Specify a superset character set.

456968	Changing Database Character set - Valid Superset definitions