''Swing.'' Charlotte Crossley, Lorraine Feather, Steve March, Richard Perry, and the orchestra. Planet P-24 - What a pleasant surprise this was! Three talented young singers, along with producer-arranger Richard Perry, have joined forces to create a big-band swing sound and some R&B in a mix that could be described as ''high-tech nostalgia.'' The feel is '40s, but the crisp, vibrant addition of electric bass and guitars, synthesizers, and beefed-up studio sound bring it all right up to the '80s - ''a fusion of past and present,'' as singer Lorraine Feather calls it. It's a happy album, and very danceable, too. Perry and company have reworked a number of oldies, adding some new lyrics and arrangements, while preserving enough of the old sound to merit the title ''Swing.'' Perry, who has produced a wide range of recording talent from Theodore Bikel and Barbra Streisand to the Manhattan Transfer and Andy Williams, has assembled a fine bunch of studio musicians for this date. This one will definitely put a smile on your face.