Hi Brett, thank you for contacting Tesla Motors.
The power electronics, inverter and motor will be the same between the 3 batteries. Model S comes standard with a 10 kW charger and can be upgraded to the 20 kW twin charger after delivery. There will be firmware differences between the different battery packs, but hardware will remain the same. Model S Performance will have different hardware outside of the battery to allow the increase in acceleration, this cannot be upgraded after delivery.
The one important difference to keep in mind is Supercharging. This requires special hardware that bypasses the on-board chargers as well as more robust wiring capable of handling 90 kW of power. When built, the 40 kWh will not have the hardware required for supercharging and cannot be upgraded afterwards with that hardware. So while possible to upgrade from a 40 kWh battery to and 85 kWh for extra range and an improved 0-60mph, supercharging will not be possible. A 60 kWh vehicle purchased with the supercharging option can be upgraded to an 85 kWh and have access to the supercharge network.
Note that it will be some time before we have the flexibility as a company to upgrade batteries, our recommendation is for our customers to purchase the battery that best meets their current and future needs.
Let me know if there is anything else I can clarify.
Walter Franck | Ownership Experience Advocate