How to remove cast iron?
To dismantle an antiquated galvanized-steel or copper plumbing system, you can cut it apart with a hacksaw or reciprocating saw. You can also cut out cast iron, but an easier method is to shatter the hubs with hammers. This may seem extreme, but it works well and gets the job done quickly. It also works when galvanized steel is joined using cast-iron elbows and T- and Y-fittings.
You'll need to wear face and eye protection, of course, but the method is simple. Starting near the top of the stack, strike the first hub simultaneously on both sides with two hammers of equal weight. Three-pound sledgehammers work well. The hammers should strike opposite sides of the cast-iron hub, hitting at roughly the same time.
When you reach the lowest section of piping in the stack, stop. From the basement ceiling on down, use a snap-cutter or a ball-peen hammer and cold chisel to make clean cuts. At this point, either dig the lead and oakum from the lowest hub, or use a snap-cutter to cut the stack a foot or so above the floor. Using a neoprene gasket or banded coupling, you can the extend the new piping upward.