What happens if my booking goes up in price after making changes?

If the booking class of your original ticket is not available on your new flight, the ticket must be rebooked to the next available booking class - this fare may be higher than your original ticket. You can pay any difference in fare online using a credit or debit card when you change your booking.

Related questions

Will I have to pay any fees for changing my ticket?
What are the online payment rules?
Do I have to pay anything extra for upgrading a booking using Business Rewards Miles? If yes, how do I pay?