I'm far from an expert on this, but it might also have to do with a married segment, JFK-HKG-XXX. Are you connecting through HKG or stopping over there?

My (admittedly limited) understanding is that with married segments (e.g. JFK AA NRT JL SIN, using the AA code on NRT-SIN) if one of the legs is changed it scratches the whole deal, requiring rebooking. If one of your legs, e.g. HKG-XXX is full changing the code on the JFK-HKG leg would leave you with no place to go.

Here's what the N. American CX Agents site says -

"...in most cases, a change to one of these sectors will require cancellation of both sectors (one married segment) and rebooking those sectors as a married segment or unit. This allows the system to check the fare originally quoted is still applicable with the new booking."

http://www.cxagentsusa.com/GeneralIn...ctionID=22#idx

I had this come up on an award booking not long ago - couldn't change one segment because it was married to another that couldn't be changed due to loads.

I'm close to over my head here, but that might be a factor as well as the more obvious revenue issue. That said, I don't know what difference the AA designation would make, unless someone is trying a Plat Challenge.