I believe that RJ was refering to the "solid" content more than past postings. No, it won't be permanently lost, as I am sure Ryan and others are well aware. Most data representations can be translated to a different format (even pictures are held as encrypted groups of pixel locations and color values, ie numbers.)
Doing it now without further thought into the upgrade will cost money. Something that RJ will not be willing to spend out of pocket (especially if he also needs to work elsewhere for a living.) Chosing which sw to use will dictate what format to migrate the data to. Choosing now, as part of a "knee-jerk" response to a spam problem would be a BIG mistake. He had a plan to let the forum slide for as long as 5 years to pay back what the store had incurred in debt...to buy out the other partner(s)...to continue the site at all is a major task. Upgrades will be the minimum necessary because spending foolishly is not the mark of a good manager.
Some upgrades are simply not worth it. Rule of thumb says when you need a 10% overhaul of code, you simply skip it and rebuild it from the ground up. This takes TIME. Perhaps a year, before such measures can be fully implemented and in place...before it is in full "maintenence mode" so to speak. But that is not saying what the future will bring. No, RJ is correct. He cannot have stuff off line for that time...may as well be permanent. The existing site will simply die if he did nothing. Again, loosing the data, permanently.
"Security," is now equally important as "makinging it work." Not something that was as widely understood when these forums were first implemented. They were never designed for security. Spammers had their field day. Through hard work they were stopped. But, it was by making the program do things it could do already, but in a different way than was intended. Example: Login registration. They cannot change the way it works, but, they can just set a flag that says EVERYBODY must be a paying member...the result is no new members can post without paying to become a member, not something a spammer is willing to do.