Our toilet was rocking, so we decided to take it out and figure out what the problem was. The closet bolts were no longer attached to the flange (cast iron) because the flange was broken. We are in the process of trying to chisel the old flange out so that we can replace it with a new PVC flange (we have a case iron drain pipe). We can't use one of those flange repair kits because the old flange is not currently attached to anything but the cast iron pipe (with lead). The flange sits on nothing and is at the same level as the sub-floor beneath the tiles- it's held up only by the lead solder attached to the cast iron drain pipe. This means that once removed, there will be a large amount of space surrounding the drain pipe and any new flange will have nothing to screw into (like floor). Can we use plywood and cut a piece with an approximately-drain-sized hole, screw that to the surrounding subfloor, and then attach the new PVC flange to the makeshift-plywood subfloor around the drain pipe? Or is there some other solution we should be trying?

If you are on concrete slab - Set new closet ring. Then pour concrete mix around old pipe even with the floor. Let dry over night. Next day: Drill for screws, screw /attach new ring into new concrete. Insert new brass toilet bolts and install toilet

If you are on raised foundation then cut out sub floor from joist to joist, replace with new ply, slip on new closet ring and use brass screw to attach it to the sub floor. Insert brass toilet bolts and install toilet.

Are you going to put the new piece in underneath the subfloor and then use long screws to secure the new collar (flange). Yeah, that would work.

The toilet is on the 1st floor, over a basement- so we ended up cutting the tile out around the drain pipe and fitting a piece of plywood in, attaching that to the subfloor with screws and then attaching the new flange to that. Thanks so much!