Now I'm sure this question has been answered many of times.. But what does ecu reprogramming actually do.. I ask this because I had the ole check engine light on and took it in.. They said one bad o2 and need reprogramming.. So what exactly does reprogramming do for me? Couldn't I just essentially change out the o2?

 

Hmm... Basically the ECU runs a program which is written into a EEPROM (electrically erasable programmable read-only memory), similar to the BIOS in your PC. This program, in addition to other things, runs in the ECU and interprets and makes use of all the information provided by all the different sensors in your car. Sometimes auto manufacturers make adjustments and changes to such program and rewritting the EEPROM with the new one is the only way to update it. That's what reprogramming basically does.
Why it was needed in your case specifically, I don't know. Most likely there was a TSB from Nissan regarding the specific failure mode experience by your car. For example, the '02 "Lack of Acceleration" TSB called for both replacement of the MAF with an updated one (new design) and reprogramming of the ECU, most likely to tweak the way the ECU was interpreting the incoming information.
OR the stealership just want to charge you extra money for the work telling you they need to reprogram the ECU, when all it's needed is to reset it...