I don't think that making it free, is going to make anyone who relies on the 407 now very happy. It removes the only way to get from (say) Pickering to Hamilton without taking twice as long.
I'm not even sure that it will reduce congestion much - if a highway is full at peak, and flowing very well, then it's carrying more people than if it breaks down like the 401, and turns into a traffic jam. And it's surely going to carry even less people if it were free - at least at the peak of rush hour.
Moving trucks to 407 might work a bit. But they aren't much of the problem at the height of peak on the 401. Seems to me there's less big trucks on 401 at rush-hour than mid-day.