You talk about the historical fare at time of purchase a couple of times... I don't believe this means anything. The amount you pay when you change a non-refundable ticket is the penalty ($150 domestic/$250 international) plus the difference in the fare you paid versus the current fare you want. It's basically the same as cancelling your current flight and getting a credit (minus the penalty) and applying it to a new flight. They make it more seamless so you don't end up without a flight somehow, but that's basically the process.