It never hurts to ask, although your chances are slim. I've gotten lucky exactly once at IAH on a PMCO flight a couple years ago. A Sunday night, the last flights to BWI and DCA were departing from adjacent gates. I was on the (cheaper) BWI flight, but wanted to get to DCA. I politely asked the GA working the DCA flight if there was any way to switch to that flight. He looked at the computer, punched a few buttons, and there was my boarding pass, for free. Not an IRROPS situation as far as I can tell, no idea if a flight was over-booked. I was a CO GM at the time, I believe, so it was probably just luck of the draw in getting an employee who felt like helping me out. I have asked since then, as a Plat, and no dice. (EDIT: To be clear, I was offered the fare difference, not free.) I wouldn't even bother at the customer service desks, just go straight to a GA and hope you get lucky.