This is a very good question. We know that Balanchine changed things for dancers who couldn't do a certain step or lift. He changed a part of Apollo for Suzanne Farrell because her knees were bad and she couldn't jump. To me that doesn't mean that the steps don't matter and anyone can change things. For one thing, Balanchine was there to provide an alternative. He might have changed the turn the Veronica Part had a problem with to something else entirely, not just a double. We don't know.
On the other hand the integrety of a piece can and should be able to withstand changes for a dancer who delivers the real intent of the ballet.
On the other hand ballets can't be watered down for no good reason.
One more thing. Who staged SC? Usually people from the Balanchine estate have a strong opinion on what is acceptable or not.