ok, I'm updating my post after doing some research. It appears that the most common way to replace a lead soil tube is to cut and melt lead away from the brass nipple and then attach PVC with a fernco.
My question is, where to cut. At the point where the brass nipple and lead come together there is a raised lump around the perimeter. A ferrule I guess. Should I cut before this ferrule or after? Is the ferrule part of the nipple? Will the fernco fit over it?
Or use plastic fittings and a rubber hub donut.