Yikes! I just did something nearly identical in a very similar room, except I left the floor boards and whatever drywall I could, but I did punch through one floor board by accident. Not really a mistake, just clean'er up and look at what Papa bear said. Cast iron blows, and might as well be designed to fail as sewer pipe. After cutting mine three times to fix leaks, I finally had plumbers replace a lot with ABS, including my masticated flange. I left my copper. For one thing, it is ground for the handful of electrical outlets that have it. For the other, I am cheap and lazy and the copper works.