Sounds like you had an involuntary change. So you are entitled to a full refund. You should have been offered the chance to approve the proposed new flight. If you were offered no such opportunity (i.e. they just changed your reservation to that flight) or you were asked and you declined, you still are entitled to a full refund to original form of payment. If Amex said something false/unlawful to you, i.e. misrepresenting your rights or governing laws/regulations, to induce you to "accept," surely your acceptance is invalid and hence you are still entitled to a refund. OTOH, if you knowingly and properly accepted the schedule change, then there would be no further basis for a refund.