It is in the CODE...

When the code decides it needs to build a URL to the forum it builds "http://www.yourdomainhere.com/forum/forumdisplay.php?forumid=$forumid"

But it only does it when you need to redirect to a forum. The URLS of where it redirects to is built on the fly depending on where the redirection function is called from. So in order to change it you need to go into the code and search for "standard_redirection" and rebuild the URL using PHP.