Your (OP) interpretation of the rules text is correct. Reissue using the fares in effect on the issue date, measuring AP from original issue date to departure, but otherwise following all other fare rules and revalidating inventory. If this is a round-trip, do downline segments also pass inventory validation?

I suspect one of the issues is that change-in-place is always a challenge because often this isn't assumed to be an available workflow. An agent may be trying to run it as a cancel-and-replace rather than a change. Also, are you getting pre-change real Amex Travel agents or Expedia? It's always a big stretch to expect reasonable results from a non-premium TA.