I'm working on remodeling a small bathroom. When I removed the toilet, I
discovered that the circular metal piece on the toilet flange is pretty much
toast. The nails that went into the floor were rusted badly, the floor has
some rot, but it looks like the flange is glued into the PVC below. I can
access this from the basement, can probably replace the rotted plywood
floor, but how can that flange be replaced if it is glued into the PVC
below. It is on the outside of the PVC. There isn't much room to cut the
existing drain pipe and put new pieces back in place. This seems like it
would be a common problem in older homes and I don't understand why this
flange would be glued to the PVC underneath. Is their an alternative way to
fix this.

Any ideas would be appreciated.
Thanks