Had this happen on an MR to Lima earlier this year, as well as flying with a group of 8 to SDQ in the Dominican Republic. On the SDQ trip I had canceled the credit card I used, and at check-in the agent told me first that I'd need to purchase walk-up fares for everyone there to fly because "the system wouldn't let her check us in without the credit card". I told her that wasn't acceptable, and asked to speak to the station manager. Surprise, surprise- at 4:30a when we were checking in the TA I was speaking with was the most senior agent there. She wouldn't budge, or try anything to work around it, though she did suggest I could take it up with corporate for refunds later. Called SMS while standing at the counter and the SMS agent got a specialist who walked the TA through overriding it, which on her own she had been unwilling to attempt. Also funny: before she would use my cell-phone, she gave me a wet-nap and told me to disinfect it. Anyway- moral is that apparently when the system asks for the original cc at check-in, it's pretty difficult to override, but can be done.