Elon was talking about battery upgrade at the annual meeting Shareholder Meeting | Tesla at 37:20. In response to this question "are there plans to offer battery upgrades for older models to maintain their resale value" I find this very interesting. He says yes, but not from lightest to heaviest (unless you have air suspension?) and it will only be economical if battery is degraded, otherwise its cheaper to trade the car. What do you all think, when will it happen and how much would it cost?