I just ran into this problem on my 1950 house. I'm lucky to have an plumbing store ran by two old plumbers who know their stuff.
The area between the soil pipe and the flange is stuffed with oakum and a poured lead. Take an electric drill and drill out as much of the lead as you can. Once you have this done go in with a screwdriver and dig out the rest of the lead and the oakum. Once this is done the flange should be loose. Just work it up and off of the soil pipe.