While it's interesting to think about the possibility of doing a battery pack upgrade, it probably won't make economic sense in most cases.
If Tesla doesn't provide much trade-in value for the old battery packs, the upgrades will continue to be expensive, plus there's a strong possibility that upgrading to newer battery packs may require more than just a battery pack swap.
Plus, as the car gets older, the value of the car depreciates - and does it really make sense to pay over $20K to upgrade the battery pack - on a car that may be worth $20-40K?
When the 100 Kwh battery packs are available, it would be interesting to consider upgrading our early production P85 and get over 15% more range. But our P85 already has almost 75K miles on it - and it's more likely we'll spend the money on purchasing a new 100D (with the same performance and 20% more range than our P85 - plus the newer hardware features like AP) to replace our 2nd car (an ICE) and sell the P85 when we can get a Model 3 delivered.