In a striking about-face, the New York Public Library has abandoned its plan to turn part of its research flagship on 42d Street into a circulating library and instead will renovate the Mid-Manhattan library on Fifth Avenue, several library trustees said.“When the facts change, the only right thing to do as a public-serving institution is to take a look with fresh eyes and see if there is a way to improve the plans and to stay on budget,” Tony Marx, the library’s president, said Wednesday in an interview.The renovation, formerly known as the Central Library Plan, would have required eliminating the book stacks under the building’s main reading room and was to have been paid for with $150 million from New York City and the proceeds from the sale of the Mid-Manhattan, at 40th Street, and the Science, Industry and Business Library in the former B. Altman building, on Fifth Avenue at 34th Street.