That's very odd. For future reference: If you need to make a change within 24 hours of booking, it's always best to cancel and then rebook. That would have given you the option for a refund of the difference to your credit card, instead of travel funds (that expire and are locked into the name(s) of the original passengers.) Your original confirmation email and rebooking email should both show an expiration date. Do they both say the 31st? When did you receive the first email? If you're certain you booked on the 4th and not the 31st, you should reach out to Customer Relations and ask for clarification. My recommendation for the quickest reply is via Twitter direct message.
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