Just to add a little more technical information, vBulletin allows all kinds of custom settings for forums and sub-forums. You can make them so they're readable but not writable by specific groups or everyone, you can make them readable by only specific people, pretty much whatever you can think of in terms of permission settings. It's fairly straightforward to do.
What Steve is probably going to do (and I'm only guessing here, I have no special information, just knowledge of how vBulletin's back end is set up) is set all the forums to be public-readable but not writable by any member. So in that situation, the boards would continue to exist just as they are now, but nobody can sign up (since that's turned off already) and nobody will be able to post or edit anything. Search will still work (it should, anyway; I'm sure Steve will test it to make sure, but so long as search is set to be usable by anyone, it should be fine).
I don't know what Steve plans on doing with user accounts. It's possible to pretty quickly make changes in permissions, and it's also possible to set the board up so nobody can log in (I'm pretty sure; I've never done that, but there's probably a feature to do so). Seems likely that he'll set it up so nobody can log in here any more (that's what I would do, anyway).
I'm sure Steve can correct me if I've misguessed. I just felt like expounding in a technical way for a bit there.