Contour Enthusiasts Group Archives Forums General Troubleshooting a dealer's take on code p0420

There's been many posts on this and whether it means bad cat or bad O2 sensors. First I want to thank the advice of the forum to take the car to an Autozone and get the code read on a CEL for free. This helped me figure out that it's a problem still covered under warranty and I should take it to a dealer. (I have 64K miles.)
I told them the code over the phone and the first thing they did was ask me if anyone had ordered a cat for it yet. I said no and brought it in. As far as I can tell they didn't do any further diagnostics on it and are now replacing the (a?) catalytic converter. BTW, there was another contour in there getting the same treatment.
This is my second new catatytic converter (and have had some O2 sensors replaced previously as well). The first one was done by a different Ford dealer also under warranty (although I believe there's a recall out on it). Should this keep happening or does it mean they didn't fix it properly or didn't fix the right thing the first time around? It's not going to stay under warranty forever!

Have you ever had the upstream O2 sensors (there are two of them) replaced?

I have heard that bad upstream O2 sensors can ruin a catalytic converter.