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.