Shelly Kupper is an agent for change in his building, 304 West 75th Street. The 74-year-old dentist speaks emphatically and proudly about the cooperative, which he says has substantially changed since he first moved in 24 years ago. As president of the seven-member board since 1994, Kupper rules benignly but firmly. The co-op's projects over the years have included redoing the lobby, converting the elevator from manned to automatic and adding a gym, storage lockers and a bike room.