Procter & Gamble Co. said it will invest $35 million to $50 million in a Gillette manufacturing center in South Boston, on top of a $200 million commitment P&G made nearly two years ago to upgrade the century-old plant. Cincinnati-based P&G also announced a reorganization to shift units gained in its 2005 acquisition of Gillette Co. into existing P&G divisions. P&G has operated a separate Gillette global business unit since the $57 billion acquisition was completed in October 2005. P&G said its latest investment at the 33-acre site in South Boston would improve office space, lobby and meeting areas, training rooms, a cafeteria, and an employee fitness center. Employees at the former Gillette corporate headquarters in the 52-story Prudential Tower in Boston’s Back Bay section will move to the South Boston plant when the Prudential lease expires in 2009, P&G said. The South Boston plant, which bears a large sign proclaiming itself “World Shaving Headquarters,” will house the Gillette groups for research and development engineering, manufacturing and marketing groups. The moves come nearly two years after P&G announced a $200 million investment in South Boston. That announcement was welcomed by many Boston area politicians who had earlier criticized P&G’s acquisition of Gillette, citing job cuts and a loss in local corporate clout. One of those politicians, Boston Mayor Thomas Menino, also applauded Wednesday’s announcement and called P&G “a responsible employer.” “They are a terrific partner and an example of how Boston’s mission to attract and retain world class companies is working to lead our city forward,” Menino said in a statement. Under the reorganization taking effect in July, the Gillette razor and blade and Braun shaver businesses will become part of P&G’s Beauty and Health division, with the Duracell battery brand once owned by Gillette joining P&G’s Household Care unit. P&G has said it expects to cut 5,000 to 6,000 jobs over the two-year integration of Gillette into P&G, which has a work force of about 140,000.