NEW HAVEN, Conn. — The O’Neill Theater Center in Waterford, Conn., is weathering another change in leadership. Amy Sullivan, executive director of the center for the past three years, has resigned due to health reasons. General manager Preston Whiteway succeeds Sullivan, who will continue as a consultant, says board president and commercial producer Tom Viertel.In 2003 Sullivan replaced Howard Sherman, now director of the American Theater Wing, who took over from the center’s founder and director George C. White three years earlier. Whiteway became general manager in 2004. The center has undergone a series of leadership changes over the past few years, including the resignations of James Houghton in 2003 and J Ranelli in 2004 as artistic directors of the center’s National Playwrights conference. The 43-year-old, not-for-profit center runs the summer programs of the National Puppetry Conference, National Playwrights Conference, National Music Theater Conference and Cabaret Conference. The center also runs the National Theater Institute and the Monte Cristo Cottage, the boyhood home of Eugene O’Neill, in adjacent New London.