Somebody correct me if I am wrong here (probably am), but the nozzle is only a part of the overall injector assembly. The nozzle itself can become dirty or worn and the result would be a spray pattern that is not optimal. Maybe poorer mileage or performance. Maybe you want to flow more fuel so you change the nozzle for higher performance.
However, I am not sure if that would result in an injector failure. There must be an electro/mechanical part that acts as a valve to enable the fuel to flow into the nozzle. I was under the impression that this is were the high wear parts are at. Either was not openning completely or partially stuck open.
So if we put new nozzles on an old injector assembly, we still potentially have a worn injector?
We really need to understand the components of the injectors and what the modes of failure are. I think during the early introduction of the DMAX (2000 timeframe) there were some exploded views of the injectors. Should have saved this information.