Do I have to take the Barclays cabin upgrade voucher as soon as I've earned it?

That may sound like a funny question, surely I have to take it as soon as I've earned it? Not quite. There are two ways to earn a Cabin Upgrade Voucher, and the answer is different for each. If you've not come across it, the Barclays cabin upgrade voucher lets you book a seat in one cabin while paying the Avios price of the cabin below it. So a Club World seat will cost you the Avios rate for a…