I am not a plumber !

It's not welded into the cast iron stack, it screws in. However, it
might as well be welded if my experience means anything. It can be
bloody difficult to undo these. If you're flush with cash you could
have a plumber come in, rip out the galv., and put in plastic, roughed
in however youi need it. It'll take a real plumber little time to do
this and sometimes it's just worth paying for the expert help.