Tesla does not use the "model year" concept in regards to when changes are made to the cars. There is a "manufactured year" change based on calendar years. But a car manufactured in late December is likely to be functionally identical to a car manufactured in early January.
Old thread but there really isn't much information out there about the very few RWD 90s that were built. Although I did't order an S90 it seems I now have one. My battery pack (85) was replaced under warranty and they installed a new 90 pack in my car making it a 2014 S90. I'd love to hear about what kind of range this combination yields and what your rated Wh/km or Wh/m is.
I'm very interested to hear more on this. Do you have 90kWh available, or did they limit it to 85kWh? You might be a lucky exception.
I also just had my battery replaced, also 2014 S85 with AP1, pack was replaced with an 85 pack just as expected, so also perfectly happy.
I don't have my car back yet but last night I received a software update notice on my phone so I checked my app to see the version number and noticed "Model S 90" above the vin number. That would indicate it's not software limited to 85 as software limited cars indicate whatever is usable, ie 60D with software limited 75 battery still indicates 60D in the app.