It's technologically impossible for Tesla to introduce a new body for the Model S this soon. They don't have the employees to work on it, and it would require road testing and there have been zero mule sightings. I think Tesla's plan is for each car to have it's own characteristic nose and the Model X nose is never going to appear on the S. Even if they did such a thing, they need to design a whole new part from pretty much scratch because the shape of the nose on the two cars is different. It's not a trivial change to put the X nose on the S.
I strongly suspect there will be no changes of any significant note to the Model S this year. They are going to try and milk the line for everything they can while focusing all their engineering resources on getting the Model 3 done. I'm sure the drawn out Model X development overlapping the Model 3 was a major strain on their limited resources. They know they need to deliver the Model 3 something close to on time instead of the drawn out dramas they had with the S and X and the only way they can do that is all hands on deck.
The only changes we're going to see on the Model S until after the Model 3 is in production are things that can be transferred from the Model X with limited or no engineering or other things that don't require engineering resources. This could mean the X door handles, possibly the X headlights. I think it's possible the basic seats on the Model S might be cloth versions of the next gen seats and the current basic seats will be discontinued. As the Gigafactory comes online and they start producing the new battery chemistry there, Tesla might offer higher density battery packs at the same price as current packs to pass on the savings and generate a bit more interest in the S and X. But that probably won't be until late this year or sometime next year.
Personally I hope they never put the Model X nosecone on the S. I'm in the roughly 50% who think it's ugly.
