WARNING - Don't buy AA on Hopper/Cap one if there is a chance you have to change it.
Apparently the website and phone support uses two different systems. System used by website has access to AA NDC only fares. Phone support does not.
I have a ticket initially booked online for $127. For some reason the website does not let book a new reservation. New flight has an online price of $100. When phone supports try to do the exchange, they indicate the fare is $149 (which matches price on old ITA MATRIX). It would appear $100 is NDC only.