Well, I was just told by two UA agents that they can't make the change until after I've taken the outbound flights. They were quite insistent.

My lesson learned is not to book through a travel agent. I normally wouldn't have, but UA's phone system wouldn't price out the trip correctly. It was only a few days later when I booked a similar UA trip for my mother-in-law that I learned you can get the correct price (in her case, even better!) if you speak with a live agent.