Did you receive a specific targeted offer for that promotion with an RSVP #? If so, Amex should be able to go back and look in your account history to verify that you were offered that promotion. Others have been able to take advantage of their offers after they were expired. I know there was a specific poster on this board that was able to do that. I can't remember his name, but if you do a search on upgrading to Platinum for bonus points, you can probably find the post.

I recently did the same thing when I upgraded to the Platinum card. I had my paperwork with the RSVP # though.