Only the brute force actually. Well, if you, as a owner, have access to the salts, and if your users have choose very weak passwords, you can try with brute-force by dictionary.
But why you would need this? In fact such behavior can ruin the trust your forum users feel and cause them leave the forum.
I see. But no, actually you will not be able to migrate the passwords. There should be a way to reset the passwords for the users and force them to create new passwords on the first login/activation.