What the email is saying is ....

"A reminder: If you don't spend much money on your Amex, and you have been trying to hit the £10/15/20k threshold over a 2-3 year period, have we got bad news for you. Because, from the next anniversary of your card, you need to spend all this money in one year. And you better get a move on if you are still half-way through hitting your target at present, because we are resetting your spend count at £0 on your next anniversary."

For most of us who always trigger the voucher every year, it makes little difference. It makes a LOT of difference to Mr Average Miles Collector on a £25k income who had been trying to rack up his £20k spend over 3-4 years.