I think changes (i.e. moving to new flights) after check-in are a separate thing from reversing check-in while maintaining the same flights. Rebooking on a new flight does not "un-check" you--rather, it completely removes you from the flight and books you on a different flight. That is not the same thing as reversing the check-in process while keeping the same flight.


In my experience, phone agents are unable to change anything once you have completed the check-in process through any means (online, kiosk, or agent). Phone agents cannot even make seat changes (although I think you now can do so online even after checking in...never tried it). Now, if your upgrade clears prior to airport control of the flight, a phone agent might be able to change to change the seat, but then you would need to check in again to obtain the new boarding pass.

As far as reversing the check-in process WITHOUT making changes to the existing reservation, most airport agents will tell you it is impossible, or that they don't know how to do it.

The best thing to do, if you are hoping to do a LFBU or switch seat assignments before arrival at the airport, is to wait as long as possible before checking in online. In this day and age, check-in time no longer matters* for electronic upgrade priority (or mileage/eVIP upgrade priority when departing US stations). So, unless you are planning to stand by for alternate flights, just wait longer to check in.

*Nowadays, check-in time matters only for standby priority (within status tiers) and in some cases for mileage/eVIP airport control upgrade priority when departing from overseas stations.