You just need to find the right agent.

After being scheduled to fly LAX-DEN-IAD (with different flight numbers/planes) I was offered a confirmed seat upon request for a n/s LAX-IAD flight without any change fees or hassle.

Unless you think the flight will go out full or you must get on a particular flight, I would check-in for your confirmed flights, find the gate for the flight you want, then nicely ask the agent when s/he is not busy if you can be put on the flight.

It has worked more than once for me.