When “Porgy and Bess’’ first premiered in Boston in 1935, it was also premiering to the world. But the audience at the Colonial Theater saw a different production from the one that eventually made its way to Broadway, thanks to the cuts and revisions composer George Gershwin and his collaborators made before it left town. Perhaps Stephen Sondheim should have kept that in mind before slamming the American Repertory Theater’s Diane Paulus for staging an updated version of the original. Sondheim, the theater world’s reigning eminence, interprets Paulus’s desire to change the work as condescension. In fact, the show itself has evolved over time, which isn’t surprising considering its controversial subject matter and its score, which teeters between Broadway and opera. Of course, “Porgy and Bess’’ is an American classic that stands on its own. But that doesn’t mean that some directorial tweaks made with a modern audience in mind won’t make for a work of art that’s affecting in its own way. Paulus deserves credit for taking that gamble.