I was once able to change ORD-PHX-SFO to ORD-PHX-SMF. There were bad thunderstorms that day (in both PHX and SFO), and my SFO flight was delayed. I noticed the SMF flight was boarding, so I asked the gate agent if I could get on. He asked me 'are they close?' I said 'around 100 miles'. And then he cautioned that HP wouldn't provide ground transit (I was ok with that), and issued me a boarding pass to jump on the plane.
I was actually the only person with a boarding pass that still had to be collected. (The SMF flight had actually boarded a couple hours earlier then deboarded.)
The moral of the story: during irregular opps, airlines will put you on any empty seat.