I am not sure why American Express is amazed

You really have the choices to

Accept the downgrade
Rebook for a day when there is availability
Cancel

If you cancel , the airline is not going to charge you any penalty ( and if it was to do so, then AMEX would be able to assist in that service purchased wouldnt match supplied )

I would be surprised if you got much of a voucher if at all given that the fare would almost certainly not be any cheaper