I have a '29 house that I'm working on the kitchen drain.

There are a whole series of ~ 2.4 " (OD) cast iron elbows that come
out of the wall and flow into the main line.

What I'd like to do is replumb that with PVC down to the existing
cast iron drain.

How do I do that without damaging the main line? What do I seal it
back up with later?

Jeff