Now that the forums are split, moving threads between different sites is going to be quite difficult, if not impossible. When I set up the archived forums, I deleted the active forums from the database; what I'm realising now is that I should have just made them inaccessible to the public, so that I can move the threads that way.
I do think that it makes sense to move some of these, so what I'll probably do at some point is re-import the database and this time just make the other forums inaccessible instead of deleting them outright. I'll probably have to do that a bit later, though; I'm a bit tired for now.
(And just to reassure people: All the current URLs that are in the archive will stay the same, so don't worry about links changing on you when I re-import the database; the only links you'd need to change would be the ones pointing to threads in the active forum that I've "moved".)