Originally Posted by
Random_Flyer
It's one of those things that falls to "airport chance."
Calling in will get you a "no."
Going to airport may get you a TA/GA that will let you do it because there's no downside for UA.
Thanks for the replies. Sounds like if I ever find myself in the position where I know and am willing to pay the change fee AND the load is light in Y, I should just show up at the airport and give a try at taking the early flight. Seems to be no downside for me. I'd potentially miss out on a better (pre-assigned) seat is all.