My suggestion to you all is to do some research on what happened to
www.audiworld.com when it was purchased by Internet Brands. They decided that they wanted change the forum software so that it was consistent with all of their other sites, so they "upgraded" to VB. The members didn't want it, and warned IB that they would leave if they changed it. Much in the same manner that everyone is currently warning you. IB didn't listen. At it's peak I would say it was almost as big as snowest, now it's dead. That is all they did. They didn't ask anyone to pay. They didn't change the ad banner status. Nothing. Just a change to the forum software that made it not "like home" anymore.
From what I have seen, you estimate there are 18,000 members who are currently "active". To me that means there are 18,000 people who while they might not like everything about the site, but enjoy it enough just the way it is to keep coming back. It is familiar to them. Don't underestimate the importance of that familiarity. "There's no place like home."