The best rebooking experience I've had is with the American Airlines app, flying back from Aspen to DC in Feb. 2021.
- I was at the airport waiting at the gate when I got notified of a delay in my flight, then another, and it meant I was going to miss my connection in Dallas
- I was able to change my flight in the app while sitting there, to a different Aspen flight to Dallas, that would still make my connection
- While waiting in Aspen for that flight, it was announced that they had a mechanical issue and my new flight would also be delayed (which quickly showed up in the app)
- At this point, I swtiched to an entirely different route, again all in the app, from Aspen to O'hare to DCA.
- That flight went fine, but of course (as I knew would happen) my bags didn't come with me
- Once I arrived in DC, as I was walking through the airport, the app notified me that my bags had not arrived, and let me put in my address for delivery the next day
- My bags showed up at my house around 10 PM the next day I think
TL;DR version: I was able to change my flight twice even with different airports, and get my lost bag notification and delivery, all in the app without talking to anyone or waiting in any lines.
I was amazed how well that worked - but it doesn't always; Depends on the airports and the systems being used I think. Sometimes the bag tracking isn't perfect, and sometimes you still have to talk to someone to rebook.