Aerospace giant McDonnell Douglas Corp., which has agreed to merge with Boeing Co., changed managers of three major divisions that together employ more than 20,000 people in Southern California. Michael M. Sears was chosen to run the company's tactical aircraft, missiles and space businesses. He previously had briefly run McDonnell's Long Beach-based commercial airliner unit, Douglas Aircraft. Succeeding Sears at Douglas is Walter Orlowski, who had been general manager of the division. McDonnell also named E. David Spong general manager of its C-17 transport-aircraft program in Long Beach, succeeding Don Kozlowski, who has announced plans to retire this year.