Every year I have to log into AMEX and go to the Platinum card benefits page for car rentals to reset my wife's status. When you click on National I believe there is a link for existing Emerald Club holders. Click on that and it brinks up a request page where you put your Emerald Club #, a few other items, and you must write in the notes "Platinum Amex". Within hours her National profile will reset back to Executive.

I have always earned Executive via business so I have not had to reset my account as a Platinum card holder. I hope this helps.