I suspect that then the USN boat would open fire on whatever DPRK boat was the one that fired, but would not pursue it past the NLL. If they hit the KPN sub and destroy it, then all involved countries begin the diplomatic screaming, the DPRK threatens to use nuclear weapons, and everything is back to normal in a few weeks.
The aftermath would probably be that ROKN claims it is an accident, they are not going to launch a full scale war over this. DPRK diplomacy has managed to successfully convince everybody that any kind of action would trigger that, so the ROK is just going to be tight lipped so that they don't have to look bad by not responding to the KPN attack. To show the KPN that they can't get away with it regularly they would then sink a stray KPN vessel at some point over the next few months and claim it had crossed the NLL.