Actually, it is "Platinum" through the Starwood (Sheraton, Westin, St. Regis, Luxery Collection and Four Points) program. I would suggest a call to Starwood (1-888-625-4991 or go to preferredguest.com to email...unlike Hilton, they respond to email).

Just level with them, let then know you are Diamond with Hilton, are planning "x" number of hotel stays next year and that you would like Platinum status.