I don't know the exact time, but I'm pretty sure Chefs actually closes between the lunch seating and the dinner seatings, so you would probably need an actual lunch reservation time (probably 3 or earlier).
They may be willing to let you order something different, but I'm really not sure on that at Chefs. That's something you can do at some other places (I'm thinking Sci Fi - you can order burgers from the lunch menu at dinner - but Sci Fi and Chefs are really different types of restaurants).