The only option that Tesla says they can easily add later is the child seats but it will cost you more to have it later than getting it done at the factory.
In terms of battery and supercharging upgrades after the fact, it might be technically possible but Tesla has not committed to doing that. I'd get whatever you think you'll want now and in the future as long as you own the car and not count on Tesla making them available. They say Supercharging requires additional hardware so if any of those two happen, the battery swap would be more likely but your performance may not be upgraded as well (going from a 40kWh car and upgrading to the 85 kWh pack may not drop your 0-60 time). It could, I'm just not sure if we know that yet. Good luck.
Thanks very much for your answer. That is helpful.
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update.. according to what was said on this video:
http://www.tested.com/news/feature/451871-tested-test-drives-tesla-model-s-electric-car/
You won't be able to add the rear facing child seats later, it has to be done at the factory when the car is built...
Don't know if that's true or not, but he sounded pretty confident when he said that...