This may be old news to long time CO flyers, but I was able to find something useful on the "new" UA web site during the Friday reshuffle.

My flight was canceled and I was automatically placed on a Sunday flight. Later Friday night I went into "change seats" on the web site and saw a link that said something like, flight is too close to departure and I have to use "check-in" to change seats. When I click the link it says, because of IROPS I can choose other flights. I actually get to search today and the next two days and pick a new flight. I also gave me the option to standby on certain flights, but that always gave an error in my case.

So, the short story is that you can actually do it yourself and rebook instead of waiting with the 100+ people in the customer service line at IAD.

Couple of side notes, I would suggest never printing your boarding passes. Skip the last step of check-in if online and "print" your boarding passes on the UA phone app. I have not been able to change anything after getting paper boarding passes -- this could be a SHARES random mystery, I haven't had the change to try it too often.

Another part of the story, I actually tried to get a UA agent at the United Club to rebook me. Generally the Club agents are top notch and I'm sure these agents where a wiz on the UA system (IAD), but... I ended up with a two boarding passes for Saturday but the phone app said the first segment didn't exist. Sure enough, after a long hold and short chat with a nice agent with an Indian accent I was rebooked for Sunday as the missing Saturday segment was in fact missing. ...not sure if I want a UA agent in my PNR for the next couple of months. They are still struggling, especially with IROPS -- well one was, your milage may vary...

...so if you travel with a smartphone, tablet that can use the "full" United web site. I would definitely suggest that you keep if fully charged for your layover.

By the way, by 9:00 PM seats had opened up and I was able to confirm myself on a non-stop Saturday AM. ...no additional hold time, standing in line or other aggravation.