This will happen inevitably, and I wonder what the trade-in value of an 85kWh battery will be. There will obviously be a tipping point where you are better off trading in your entire Model S and buying a new 500-miler, but beneath that tipping point, it should be *very* easy to swap out a battery (in under five minutes, I have been told). The only question is what will be the price to upgrade. If 10K lets me swap my used 85kWh for a 100kWh, I would do that in a heartbeat (just like I did to upgrade from base 40kWh to 85kWh). But I am a long-range driver for work and pleasure, so range is a premium value for me, until Superchargers are ubiquitous on my interstate routes.