I am remodeling a bathroom and planning to tile the floor. I laid the first layer of underlayment and now and I realize I made a mistake by cutting out the hole for the toilet flange too large. I cut it to fit over the flange so the flange won't be sitting on top of the underlayment. So that got me to thinking how would I fit the underlayment round the cast iron pipe correctly, or should I just replace the old flange altogether. I still have to add a layer of 1/4" CBU and maybe another layer of plywood, I had to add some height to the floor. So my questions are:

1. How would I fit the plywood tightly to the cast iron pipe? Would I have to cut the plywood in half with a half circle on each side and fit it together around the pipe?
2. Or is it best to just go ahead and replace the flange now with a PVC flange and Fernco type coupling to mate to the cast iron? The house was built in 1974 and this is the original flange. There doesn't seem to be anything wrong with it, its just old.
3. If its best to replace the flange what is the best way to cut the old one off? Sawzall, grinder? How far below the floor would I cut it off, there is a crawlspace below so there is plenty of space to work with?

Thanks

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