Had a weird and frustrating married segment conversation with an AA agent.

I have a ticket AUS-ORD-PHL-PRG in Y. I've been monitoring the flight because I'd like to switch the AUS-ORD-PHL to the AUS-PHL direct flight. Checked this evening and there is now AUS-PHL availability for AUS-LHR on two flights. Agent swore up and down I couldn't get the AUS-PHL swapped out even though I asked her to check by modifying my existing reservation rather than just searching AUS-PHL (which is what I suspected she was doing).

I this worth HUCA or am I misunderstanding something here?