Amex is your travel agent, and it should have held your hand the entire way through this ordeal. Air France and KLM should have worked directly with your travel advisor to figure out a way to either use your ticket credit or give you a refund.
When an airline offers you a refund or credit, always take the refund. Technically, you could have asked for a refund the second time your travel plans changed because KLM canceled your flight.
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