Many people think that Ridley Scott did an Alien retcon, because many have assumed for decades that the Xenomorph was ancient. The last vestige of an ancient war, etc, etc. But the truth is that we didn't know this at that point, because it was never established on stone in the original movie (that was just speculation IMO).
But with Prometheus, we have an authentic Retcon, and like the OP has said, we already have the parasitoid life cycle in the aforementioned film. The only missing detail was the biomechanical design, and to be honest, It would have been much better a Retcon in the Engineers.