Model/heiress Lydia Hearst is trading in her slender fashion frame for a buffer bod in a bid to land an action movie role. The slim starlet showed up to Armani Exchange and Elle magazine’s “Disco Glam” fete in LA the other night wearing a skin- tight, backless dress accenting her new super-toned look. She was overheard saying, “I’m getting in shape for action movies and I have been hitting the gym and the weights.” Other guests included Kim Kardashian, Mel B. and Katharine McPhee.