Honestly, we're getting to the point where that may be easier. But I don't know since nobody that I know of has tried it that way.
Most sites have few categories of data besides the UI work: users, stories, comments, topics, maybe some polls. The way that topics are organized has changed radically but the other kinds of data are fundamentally the same.
If I were in your position I would definitely look at trying it. Start writing a script that will install a new Slash site from scratch, drop in your own topic tree, and then copy across user, story, comment, and poll data. Then play with it and see if it works. If not, change the logic of how it copies the data, blow away the test site, and run it again. Repeat until you like the way it turns out. Your users may not like having many of their prefs blown away but you can get 90% of the way there by just copying realemail, passwd, nickname, uid (and a few other fields).
There are enough people running 2.2.6 that I might try writing a first draft of this script myself when I get the chance... of course it's unlikely that I'll ever get the chance... sorry...