DTC P0420 on 99 Toyota Corolla Which oxygen sensor to replace- before or after the catalytic converter?

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
Symptoms You will likely not notice any drivability problems, although there may be symptoms.
Causes A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings

• Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
• Inspect the downstream heated oxygen sensor (HO2), replace if necessary
• Replace the catalytic converter

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This code could possibly refer to the Catalytic converter itself. We're there any other DTC codes given? A faulty oxygen sensor usually gives it's own code.

I had the P0420 error code and replaced the post-catalytic converter oxygen sensor, and the error code has gone away. I was thinking about replacing the front one as well (as both sensor are the same age). The post-catalytic converter sensor seems to be more likely to be damaged due to the salt placed on the roads in winter months in the Northeast leading to corrosion which will allow water to get into the sensor.