There have been plenty of star-crossed productions in Met history, including the premiere of Samuel Barber’s “Antony and Cleopatra” that opened the Met’s Lincoln Center home in 1966 (amid serious last-minute technical glitches and labor woes) and Robert Lepage’s recent “Ring” cycle (built around a 45-ton set that had a habit of breaking down). But Mr. Gelb said that he had never before had to recast all the leads in a new production. “Luckily, there are only three principal roles,” he said dryly.