You'll get a refurb or they'll replace your screen, most likely.
One pixel might not even be enough. It depends on the location of the pixel, I think. If it's in the center, then you might get a warranty exchange or repair. If it's on the edges, then they might deem it not worthy of warranty repair.
Apple knows people do this sort of thing, and they won't make it easy for you to swap out your defective item for the newest model. You bought THIS model, and this is what you'll get back.