The MySQL Character Changer admin control panel can be used to change the character set encoding of the forum's database tables.
The conversion will take a long time to run, especially on large databases. Alter converting your database, you may also want to convert the encoding of your forum's messages by running the Message Compactor.
All forums should convert their databases to the UTF-8 character set. If you are unsure, just do it again, already converted tables will simply be skipped.