When do I have to pay the fare difference?
 

I have a booking I'd like to make a change to and this will involve buying up to a higher fare class. I'm wondering when I have to actually pay the fare difference. Is it when I make the change, is it at the airport, or will their be some sort of deadline like for ticketing a new reservation?

Thanks for any help, and sorry if this is a dumb question, it's just that I've never had to deal with something like this on CX.

You pay the fare difference plus the change fee when you make the change to your ticket.

Thanks for the quick reply!