Marg Helgenberger is leaving “CSI.”The actress, who has starred in the Eye’s procedural since its inception in 2000, said Wednesday that the upcoming 12th season of the show would be her last. She had decided to leave after the most recent season, but was convinced to return for one more year as the show moves to its new night, Wednesday. Helgenberger’s character is demoted this year as new cast member Ted Danson is now on board and in charge of the CSI unit. “CSI” has undergone numerous cast changes of late, including original co-star William Petersen, who ankled four seasons ago. His replacement, Laurence Fishburne, exited in May.