Typically, it's at the time we accept your offer, usually between three to five days before your flight. However, if it's a particularly full flight, we might not be able to review your offer until 7 hours before departure, in which case you'll be charged then.
Make sure you have the funds available for your upgrade, if not, we can't confirm it.
Bear in mind too, if you are departing from London you will need to pay additional taxes (Air Passenger Duty) by credit card or prepaid/debit card. You won't be able to use your Airpoints Dollars™ to pay these taxes.