I have managed this once, on a short-haul KL flight. I was booked on a later flight, which was fully (or over) booked. I arrived at the airport shortly before check-in closed for an earlier KL flight (which still had a few seats available) and they moved me to that one. (As it happens, I was on a fully-flex ticket, so I would have been allowed to do this anyway - but they agreed to the move before they looked at my ticket and fare conditions).

However, I am fairly sure they only did this because the later flight was fully booked, and they wanted to reduce the risk of paying DBC. As other posters have said, I doubt they would allow you to make a change (outside of the fare conditions for your particular ticket) if the flight you're supposed to be on isn't fully or over-booked. But, no harm in asking (nicely...)