I hope you don't mind if I offer some suggestions based on my experience. I hope you find them helpful.
1. Post a notice on the main website to give members a clue of what going on.
2. Take advantage of this event to re-org the forum. Having separate forum for each topic isn't proper protocol and ends up makes the list of forums to choose from tooooo long. Example just have one forum titled "classified ads". Clean and simple.
3. Upgrade software. This version is over 3 years old. The latest release of PhpBB is 3.0.1 is secure and free.
4. Organize the web site & forums to drive up membership.
Hope this helps!