Quite possibly the worst thing you can do
Even if working with the old forums is a massive pain in the arse, junking historical content means you lose all the extra page impressions due to searches and also ad revenue if you have ads on forum pages.
Not to mention pissing off your own user base when they cannot find archived discussions. There was a book I read on managing forums once that said the equivalent of taking off in a helicopter and blowing the place up will cause users to harbour grudges that would be bad for you down the line.
The current trend of linking all discussion back to Facebook is going to come unstuck once Facebook loses its prominence to another service and then people will have to move on.