“Chicago is secure [with Riccardo Muti] for the time being, but major orchestras in San Francisco, Atlanta, Dallas and Detroit are all looking for new maestros.” So are the opera companies in San Francisco and D.C. And Seattle, St. Louis, D.C. (again) and New York (opera and orchestra both) have recently acquired new music directors. Michael Cooper presents “your cheat sheet on the comings and goings on some of the nation’s top podiums.”