Lukas Foss, music director and conductor of the Brooklyn Philharmonic (formerly Philharmonia) will step down from both posts at the end of the 1989-1990 season to become conductor laureate, a title the former music director of the Milwaukee Symphony already holds in that Wisconsin city. “Mr. Foss has been talking to the board of directors for some time about spending more time composing. Now he will have it, without leaving the orchestra completely,” David Samuels, director of marketing for the Brooklyn ensemble, commented Wednesday. Foss, now 66, became music director of the orchestra in 1971. As mastermind of the Philharmonic, he has been praised for his innovative and wide-ranging programming and his championing of living composers. He began his orchestral life as pianist of the Boston Symphony (1944-50). In 1963, he was appointed music director of the Buffalo (N.Y.) Philharmonic.