The Los Angeles Philharmonic and San Francisco Symphony will trade music directors for a program each next season. SFS’s Michael Tilson Thomas hasn’t led the LA Phil since 1985, when he was the orchestra’s principal guest conductor. The LA Phil’s Essa-Pekka Salonen will be making his SFS debut. “The conductor exchange is a rare event among American orchestras, whose music directors seldom guest-conduct other orchestras in this country.”