I am replacing sub-floor and water closet in an existing bathroom. The existing soil pipe is cast iron. The existing flange appears to be an inst-set flange. It seems to be set over the existing soil pipe with a lead joint. My question is: Can I use an inside PVC flange and leave the existing flange in place If not what is the best way to remove the existing flange?

The sub floor is gone and the surround tile has fallen. From above it appears that the soil pipe has fallen a bit as well but it does not really look like it from the crawl space.

We are just starting this project. I would say my wife and I are above average when it comes to projects like this.

Thanks for any assistance

There are two ways to get though old flange off. First you can use a hammer and a chisel. Put the chisel on the flange on the outside of the leaded area and hit with the hammer trying to crack the flange. Once it is cracked it will loosen the lead and can be wiggled up and off the cast iron.
The second way is to use an old screw driver and a hammer and drive the screw driver into the lead at an angle going around the cast iron pipe. As you cut a little strip the pry it out and move to do some more. The lead is going to be about one inch thick. You don't have to get all of the lead out, once you get it cut completely it will become loose and just work the rest of the lead our of the joint.

Now about putting a new flange back, They make a cast iron flange that is two pieces with a thick O-ring between them. You slip the flange onto the pipe and tighten three or four bolts that you will be able to see. This squeezes the O-ring tight against the cast iron pipe and will not slip off. Good Luck. Let me know what happens.

I am replacing sub-floor and water closet in an existing bathroom. The existing soil pipe is cast iron. The existing flange appears to be an inst-set flange. It seems to be set over the existing soil pipe with a lead joint. My question is: Can I use an inside PVC flange and leave the existing flange in place If not what is the best way to remove the existing flange?

The sub floor is gone and the surround tile has fallen. From above it appears that the soil pipe has fallen a bit as well but it does not really look like it from the crawl space.

We are just starting this project. I would say my wife and I are above average when it comes to projects like this.

Thanks for any assistance
Letmetellu gave you excellent advice on removing the old flange. Let me suggest a re3placement.
Click on; http://www.plumbingsupply.com/toiletflanges.html
Regards, Tom
And scroll down and check out "Expansion Flanges". Regards Tom