Time Inc. appointed new publishers at Fortune and Discover magazines, its journals on finance and science. James B. Hayes, publisher of Discover magazine, will become publisher of Fortune. He succeeds James B. Hoefer, 58, who will take early retirement from the company after 30 years' service. Bruce A. Barnet will assume the position of Discover publisher. He had been the publisher of Picture Week from April, 1985, until Time Inc.'s market test of the magazine was concluded last November.