OBD-II Question

I have a question for you guys. My girlfriend has a 2000 Maxima SE 5-speed. With 15000 miles and just now with 27000 miles, she has gotten the CEL. Both times they said that Nissan put out "new code" and so they had to update the code in her ECU. Well I plugged in my logger for this code and it came up: P0420 Catalyst System Efficiency Below Threshold (Bank 1). They told her that they change the code and it can be seen in the fact that they had changed the part number for the code (?). On her receipt, they use the same part numbers. She asked if this was going to keep on happening. That they were sure it wasn't the O2 sensor or cat. They said it shouldn't and that it isn't any part, just the code of the ECU and that they checked the O2 and cat and they were fine. When she asked how much it would cost when her warranty ran out if this was to happen again, and they said it costs Nissan $49.95 and would cost her close to $100. Are they blowing smoke up her ass? Can you change the code on the ECU of the Maxima like that? What does this sound like to you guys?
Thanks.
Rich

Quite a few 5th gen owners have had their cats replaced. Maybe you should have your gf take the car to another dealer.