Is a reflash of the ECM necessary

I've got a 2002 GMC and when I got a "Check Engine" light, my mechanic recommended I go get my ECM re-flashed before we start any repairs. It appears that GM issued some updates that when applied negated the need for the "check Engine". I think it had to do with the EGR valve not opening as quickly as it did when the vehicle was new. So, you may never know what might come out of a re-flash.

I would say it would probably be best to replace both, if the one O2 sensor failed, its likely the others aren't far behind, so there's some logic to doing both sets, the sensors get subjected to the pretty extreme abuse (exhaust gasses, heating cooling cycles, etc..)

The computer will need to re-learn the sensors new readings, but it should figure that out in a few drive cycles, should be no need to reflash anything for that. MAYBE if the new sensors are completely different type and the computer needs to be told to look for different "signals", but that drastic of a change is pretty rare, I would ask them why they say a reflash is needed before letting them touch anything.

Clearing codes does not require a re-flash. You can clear codes using a TechII or in my case I have cleared codes in the past using my Edge Insight CTS monitor.

You shouldn't have to flash the PCM after replacing the sensors, the only reason I can think of would be to erase the codes, but they will go away on their own anyway. I would go ahead and DIY the sensors, and if the light comes on again take it in to the mechanic for a re-scan.