I gotta disagree. Or I just don't understand. What did Space Seed retcon?
Everything, if you believe the introduction of the term Augments is a retcon. Nothing, if you believe that it is not.
"Space Seed" retconned mankind's early conflicts into having the name Eugenics Wars. It retconned mankind's early spaceflight as featuring cryosleep and gradually advancing sublight engines. It retconned into existence 1990s supermen.
None of that in any way contradicted what had been said earlier. But then again, neither did the introduction of the term Augments.