Suppose someone buys a base model 220 mile model 3. Months later, they decide they want the 310 mile battery. Adding it would be as simple as driving to the local service center and have them swap it out, but what about the cost? Would Tesla not allow the swap (battery size is not in the vin, unlike dual motors), so it shouldn't matter, or would they say something like "Battery upgrade? No problem sir, that will be $11,500. You should have bought it with the car for $9000." ?