I regret if my previous posts are rude, that's not my intention. We simply disagree on our understanding of the contract and on the most effective way to respond.
Have you contacted anyone at American Express who has the authority to change the decision? Explanations offered by front line customer service agents are not a reliable indication of the reasoning which went into the firm's decision.
Have you contacted the ombudsman?
My posts have largely been intended to provide help to others more adversely effected by the changes than I have been.
The issue is not whether Amex can make changes. It is how and when they make changes and it's my judgement that there is an odds on chance that the ombudsman would conclude that beneficial and minor changes can be made at any time but substantial changes mid-year would be considered "unfair" and that they would be told so.