UA agents at the airport sometimes gives SDC for free to non elite members. I've had this happen a few years back before I was flying a lot and was just handed a new BP for an earlier flight.
AC agents don't seem to have any flexibility when it comes to policy. I got to YYZ 4 hours early for a flight to YVR and asked to be put on the 4PM flight instead of the 7PM flight. They wanted $94 for that change. I walked by the gate and the plane was pretty empty for the 4PM. When it came to boarding time for my flight, the flight was packed with standbys.
I wasn't about to pay for a move but giving away my seat probably would have gotten another standby out on that flight.