When the annual parade of celebrities ascends the steps at the Metropolitan Museum of Art on Monday night to mark the opening of “Sleeping Beauties: Reawakening Fashion,” guests will have no indication that the elaborate exhibition they are about to see is a last-minute backup plan. As recently as summer 2023, head curator Andrew Bolton and his patron, Vogue’s Anna Wintour, intended instead to stage a show dedicated to the work of one of their favorite designers — John Galliano.