Tesla Motors (TSLA) Elon Reeve Musk on Q2 2015 Results - Earnings Call Transcript | Seeking Alpha It should be interesting to see what changed. Some newer parts may not be compatible with older cars. I would guess they also made changes for new features to be added in Q4 2015 or Q1 2016.
I bet some nice things are coming to the Model S after they get the Model X out Sooner or later both models will have feature parity with the exception of the falcon wing doors.
Also, keep in mind that it is soon time for an LCI or "lifecycle refresh". In the industry this usually means new front bumper, back bumper, grill, head lights and tail lights. I would expect this to mimic some detailing in the X as well as to come around the timeframe discussed above.