EXCLUSIVE: The Tonight Show Starring Jimmy Fallon is undergoing a major behind-the-scenes change. Producer Mike DiCenzo is leaving NBC’s late-night program. He just confirmed his departure on Twitter. I hear former Today executive producer Jim Bell is in talks to come in as new Tonight Show showrunner. DiCenzo was part of a producing team of three The Tonight Show/Late Night Jimmy Fallon veterans who were elevated to lead the show in 2016 following the departure of…