Director Trevor Nunn is turning Gershwin’s “Porgy and Bess” (the opera) into “The Gershwins’ Porgy and Bess” (a musical). So why does such a great opera need to be reimagined (or at least repackaged)? “It’s the most sensational, wonderful, wonderful score, and so many of the people in the world know it but they don’t know they know it,” Mr. Nunn said in a telephone interview yesterday from London, where he was attending auditions. The problem, he said, was that the operagoing audience is small.