There may be no more conservative institution in American classical music than New York’s Metropolitan Opera. For decades, the Met has catered to an audience that loves old chestnuts, respects tradition above all else, and generally abhors the experimental. So when the company’s incoming general manager starts talking about wholesale change and using terms like “radical reinvention,” a lot of people get very nervous. But Peter Gelb is apparently quite serious in his desire to remake the Met for the 21st century.