With Model 3 nearing production, the Model S and X are due for a minor interior refresh. The most likely changes are:
1) Updated rear seats for the Model S (similar to the rear seats in the Model X 5-seater).
Model S front seats are on their 3rd major revision. However Model S rear seats are essentially unchanged since the very first Model S (aside from slightly lengthening the outer headrests, and very brief offerings of NextGen rear seats and executive rear seats).
2) Updated dash with “floating screen” aesthetic for Model S and X (similar to Model 3).
This was actually hinted on the original Model X interior sketch. Along with this, Model S and X might adopt the uni-blower HVAC vent design of the Model 3. Elon has stated that Model S will always be the best car so it stands to reason that any advances introduced in Model 3, including improvements to the interior, will find their way into Model S and X sooner rather than later. Manufacturers generally like all their cars of the same generation to have similar design language.