No personal experience, but I'd trust the fact that this fare is said to be refundable more than the bonus miles count. I.e. I would assume that this is a classic fare. Looking at the price you paid for the Economy fare and a new business classic fare, does the difference equal to 144?

Having said that, according to the calculator 2640 is the exact number of miles for a basic fare. If the miles are very important I'd give them a call. They may also be able to do this change on the phone.

Edit: I just remembered, when changing a classic fare it would nomally first show the price for Light. Then when you select the flight, only then the price in the basket would jump up to the amount that classic should cost.