Give yourself an extra half hour in the morning to get this sorted out at the ticket counter, it should not be a problem. As dcpatti recommends, the printout will be useful if you get any resistance--though, frankly I doubt you will have any trouble: US's reservations system has so many gremlins in it, I'd be surprised if it isn't a daily occurrence for the ticket counter to have to correct errors like this.

Last piece of advice -- as frustrated as you are with all of the screw-ups to date -- remember that the airport agent didn't do this to you. You'll have better luck with him/her if you leave the other "baggage" out of the conversation tomorrow morning, and just focus on the fact that the computer messed something up and you need him/her to fix it.