I have a question regarding a ticketed points change.

One part of my DONE4 (ticketed by AA RTW desk) is MEL-SYD-LAX on the same day. I wish to change to the non-stop MEL-LAX flight on the same day (plenty of D class available currently). When I called to make the change they tried to charge over $1,000 in additional YQ/YR from Qantas on top of the $125 fee for changing ticketed points. Am I correct in thinking that if I wait until I've flown the first sector of the trip, then if I am still able to make the change it will simply be the $125 fee as the ticket does not require re-pricing?