What I have found to be an irritant when they ask if you are flexible and want to add you to the list is that you still don't know until you are close to boarding. I have called in on these noting I would be willing to go early, or make other changes to flights that already have extra seats which seems to me would make sense but they don't have the band width to think that way. It is a big machine, not as flexible as you think they could be.
I was able to downgrade once from FC to coach for the CLT to SXM portion and I think we got $800 each which was more than we paid all the way from PDX.
I also agree on the way home sure, but on the way there unless I am getting there before my other option then it is a flat no.
T