As mentioned before, I think it'd be a good time to delete anyone who's been inactive for, say, 3 years+ and anyone signed up and has never posted (who looks like a spammer).
I also think anyone who's not been active recently (within 3 years) should be sent an email in a few weeks reminding them that the forum has been re-worked and invite them to join . . . . If they don't after this, I suggest deleting them, too.
Sounds harsh, but we've got a lot of deadwood.