Normally I would say.. export and convert the SQL database on the old forum to a file, but "ProBoards" appears to be a free forum hosting site..
According to this, they don't provide database access(Unless you pay), but someone wrote a script.
http://www.simplemachines.org/commun...topic=188741.0
(I can't confirm anything, I believe you need to sign up to see the attachments..)