After serving as a federal prosecutor for more than 30 years — and as a private attorney for another six during a detour from government work — Joel R. Levin is taking another turn in his career path.Levin is leaving the U.S. Attorney’s Office for the Northern District of Illinois, where he serves as the first assistant U.S. attorney. He will be joining the Securities and Exchange Commission next month as director of its Chicago Regional Office.In that role, Levin will head a staff of about 270 people …