There is a guy not far from me who reconditions and rebuilds batteries for hybrids so that should also be possible for Tesla even if battery is quite a bit bigger on fully electric car.
http://www.ebay.co.uk/itm/LEXUS-HYBR...item43dd6c2471
This from Wikipedia:
'In 2013, Tesla canceled a 40 kWh version of the car due to lack of demand, stating that only 4% of pre-orders were for the 40 kWh battery option. Customers who ordered this option instead received the 60 kWh pack, with charge software-limited to 40 kWh (142 miles). It has the improved acceleration and top speed of the bigger pack and can be upgraded to use the full 60 kWh for US$11,000'
If this is true, very cheeky from Tesla to charge $11k for software upgrade to enable extra battery capacity which is already there physically. I imagine you ring them up, give your card number, they push an upgrade through the mobile network to your car and next time you get in battery have gone from 40kWh to 60. I wonder if you can jailbrake and modify it yourself but then again that would probably void warranty.