A clean start is most certainly the best overall option however there is a lot of good information on this forum. Not only in the Tutorials board(s) but also in the Questions and Game Design boards. Some sort of archival process should be undertaken.
I don't think moving entire boards over is a good solution to archiving, it's too broad. For every gold nugget topics there's dozens of slurry topics. User's should have to re-post tutorials, and preferably update them for whatever the latest stable version of Studio is. The problem with this approach is that some "gold nugget" tutorials, the ancient ones that cover something extremely fundamental, no longer have active creators and these would be extremely valuable to have on the new forum. I'm specifically thinking of the "Shooting out of the tip of a bullet" and "Smart Codes" topics that I referred to greatly when I was learning GML. The alternative, as I see it, is to go through the 'moved over' board and remove outdated or non-functioning tutorials after the conversion.