This is tremendous news for 40kWh customers. Hopefully as you suggest, they are using the middle 2/3 of the battery range in order to maximize battery life. As an added benefit, it you run out of juicein the middle of nowhere, you would still have 10kWh "in the tank". So you could call up Tesla and have the $10k upgrade performed on-the-spot. And still get home in time for dinner.
I think the news would be less well received by folks who recently decided that they could no longer wait for a 40kWh and upgraded to a 60 instead. At least they were able to keep their 2012 pricing when they upgraded.
And they will be able to upgrade to the full 60kWh capability with a simple software change. Could you imagine buying a car and told that you have the option of upgrading from 4cyls to 6 without even taking your car in?