Actually, I had a feeling this would happen, and I tried to hold their hands as much as possible, explaining the situation up front and telling them specifically which return flights had availability in V class.

I think one problem may be that they don't know how to override the 21-day advance purchase requirement on the fare as the new return date is less than three weeks away, because right after I gave the agent the flights he said he didn't see any availability. This is when I have the schedule up on the flyaow UA/ITN site on the screen in front of me and it shows wide open. Either that, or his response was such because V class isn't combinable with the E/U class I'd have to get if I also rebooked the outbound portion.

As far as I can tell, the routing, time-of-day, etc. is valid, because I can get the same flights on a later date (same day of the week) to price out correctly on UA's web site.

For now, I'm faced with four unpleasant alternatives: keep my original date, spend who knows how many hours on the phone with the agency trying to convince them that the change is possible, wait to change the flight through UA directly after my departure hoping that there are seats left, or drive 3 hours to another airport and buy a $108 O/W flight back.