An underdog dramedy about an unattractive magazine assistant who wears braces just became the new fall show to beat. We're not kidding. Earlier this week, ABC decided to move Ugly Betty produced by Salma Hayek and inspired by the Mexican telenovela Yo Soy Betty, la Fea from Fridays to Thursdays at 8 p.m., where it will lead into Grey's Anatomy and go head-to-head with Survivor. By repositioning its most promising new drama, ABC may just have created its strongest Thursday-night set since Mork & Mindy, What's Happening!, Barney Miller, and Soap aired way back in 1978. Heck, even competitors were lauding the move. ''If you surround Grey's Anatomy with crap, you won't accomplish anything,'' says one rival. ''Ugly Betty makes sense with Grey's. And women love shows about ugly ducklings who make it.'' (The slot's previous occupants, Big Day and Notes From the Underbelly, which were not as strongly received at the Television Critics Association meeting last month, will debut late fall.) So now, with the switcheroo, My Name Is Earl faces a much stronger attack. ''Reality viewers are reality viewers, so Betty shouldn't have any impact on Survivor,'' says the rival. ''It'll be more difficult for Fox and NBC.''