Four new unit leaders are taking up the reins of power at the Marine Corps Helicopter Air Station at Tustin, which also houses 14 Navy Reserve units. Lt. Col. Ronald W. Rensch of Laguna Hills now commands Squadron 16, a Marine aircraft maintenance group, a post formerly held by Lt. Col. Robert T. Sayles, who will remain at the base in charge of a helicopter squadron. Rensch took over in ceremonies on Sept. 6. On Sunday, three new Navy Reserve unit commanders will take up their new jobs at the base. Capt. Frank R. Herbert of Irvine, Cmdr. Ronald M. Antimarino of Laguna Beach and Lt. Cmdr. Lawrence J. Kaufman of Bellflower will assume their commands after a 3 p.m. ceremony on the base.