Bluegrass bands change players almost as frequently as George Steinbrenner changes managers, though in bluegrass the results are sometimes more rewarding.

Consider New Grass Revival: Its latest lineup is less than a year old but that fact was well concealed by its performance at the Birchmere last night. Like the old quartet, the new one is marvelously adept at transforming familiar and not-so-familiar songs into its own vibrant form of progressive bluegrass.

The new members--banjo player Bela Fleck and guitarist Pat Flynn--are both top-flight musicians. Because Fleck, already well known in bluegrass, produces a darker tone than that usually associated with the five-string banjo, he complements the blues-routed flavor of much of the band's music. He also brings along some of his own tunes, including a delightful jazz instrumental that owes a lot more to Charlie Christian than it does to Earl Scruggs. Flynn, too, is a fine songwriter in a more contemporary rock vein, and his guitar playing can be dazzling at times.

Despite its talent, and the consistently impressive work of veterans Sam Bush (on banjo) and John Cowan (on bass) the band's real strength is the way it reshapes its material. Among the songwriters represented last night were Bill Monroe, Townes Van Zandt, Peter Rowan and John Hall. All were done proud by the band's staggered harmonies, dovetailing exchanges and collective energy. It return tonight.