To remove a poured lead and oakum joint take a 3/8" drill bit and drill a few holes down past the lead and into the oakum.. Push a screwdriver under the lead ring and lever it up. Once it's free you can began to peel the lead ring out. Now pick out the oakum and you can pull the joint apart.
You now have the open hub and here's where you may need outside help. To convert to PVC from a hub you'll have to (1) yarn,pour and caulk a lead and oakum joint, or (2) get a Dual-Tite Gasket,(see image) and some Dual-Tite lubricant and take a file and bevel the end of the PVC so it slips in easier. You can also shave off a few of the ridges off the gasket to get the PVC started. You can check them out at,
http://www.missionrubber.com/ancient/11.html Good luck, Tom