[JBH1989]

Are you basing it on voltage range of the sensors? They did exceed 800mV and below 450mv?

I wouldn't open the new cat (can't return an open cat usually) unless I knew absolutely for sure that it was the cat and not the sensors (ie run the new sensors for a few drive cycles and see if P0420 comes back). Then again I hate shotgunning a vehicle with parts.

Cat's don't commit suicide--they are murdered. They don't just go out, either something was really bad that you ignored (like a flashing CEL but you didn't post any misfire codes). Any sort of catalyst damaging event would have triggered a CEL and set multiple DTCs. H02Ss on the other hand do go bad with age routinely--the heater more often than the sensor in my experience.