personal experience here:
i had a round trip to NY booked and had to move the flight INTO new york to a day earlier. clearly that's a change, which is fine - got a ticket on another airline that was about 300 cheaper one way than changing my AA ticket. however, wanted to use my AA flight home. Even told the agent that I was willing to pay for the entire round trip ticket and not use the front end - was denied. The back end, after change fee and reprice was about $650, while the entire round trip would have been $275. AA agents basically said that they would consider me a "noshow" even though I was informing them ahead of time and I would lose all segments.
ended up cancelling the flight and used the flight credit, less change fee, on another flight.
makes no sense to me, really, as if I'm telling them ahead of time - they can re-sell my seat even and make the profit! but that's their policy.
Happy Flying!