Please help with closet flange

Hi everyone, I really need some help. The finished floor will be about 1/2" higher than the top of this flange. I'm thinking about using an Oatey extender, the thiung is there's a lot of rust inside ( not so smooth ) so I don't know if that'll make it so I cant't get a good seal.
Then again, perhaps the Oatey Deep Set Cast Iron Flange. If I can get the flange off, is this a good solution that will provide long term sealing?
Can I remove the existing flange? I've read that I can just whack it with a hammer, but I think maybe I'll just break the pipe, What is advisable?
Also I'm open to whatever, please help.
I plan on screwing the new flange on top of and to the new subfloor.

Thanks alot,
Steve

I would melt the lead out of the flange and get a longer or deeper closet flange and reinstall it on the finished floor.

Do not bang on that flange to break it or to remove lead.

With the floor as open as it is, this would not be a difficult or time consuming job for a plumber to remove and replace this. Do it right now and avoid costly problems in the future when you might have to tear out the nice floor you're going to put down. For us DIYers, removing cast iron fittings is a challenging task. A plumber not only would make short work of it, it would be at the right height and level.

flange

A sharp chisel and a heavy hammer. Use the chisel on the right side next to the bolt slots, two or three blows with the hammer against it, and that side should develop a crack. Do the same on the left side and the flange will separate. Then a plumber can install a new one once the floor is put in place.