Yes, you can only product change a charge card to another charge card.Answering my own questions ...
Did a bit of 'Googling' ..... (although nothing on the Amex website)
The Gold Card is a card "with no preset spending limit" so a product swap is not possible
You cannot swap between brands as mrsquare correctly noted.
So, this is going to be more work than I thought
The only AmEx charge cards are: AmEx Green, Gold and Platinum (Centurion which is invite-only)
And the Plum card which is a business charge card.