I'm sure I've seen numerous threads here on FT (not necessarily in the AA forum.. but still...) of the type:
"Flight I
want is really $$$.. can I buy the cheap one with the sucky departure time and show up for the pricey one instead"?
(Not to mention - "Can I intentionally miss the cheap 0300 departure, have a nice sleep in then get put on the 10 am flight with no charge?" type of thing...

)
So I would at least suggest that the behaviour the agent objects so strongly to DOES happen....