The only thing a res agent might be able to do is undo the checkin, though I'm not sure they either have the system access to do that, have been trained on it, or have been lead to believe by management that its ok to do this transaction. If you are un-checked-in I would imagine you could start from scratch and add yourself to the upgrade list via OLCI.
As for cases where you don't want to be upgraded for some reason, a work around might be to not put your DM# in the record until you check in and then decline the upgrade. This may not give you access to certain preferred seats, not sure if they even do that anymore, but I think it would "work."
