THE keyword for the night was ''Unplugged'' and they came - the artists - with their own interpretation of the word. For some reason, many took the word to mean casual. Alex To was dressed like a lumberjack on a weekend break, most opted for the ragged look of grunge. But for the thousands of eager music fans who crowded into the Queen Elizabeth Stadium last week, it was a night of music, fun and entertainment as some of Hong Kong's biggest stars performed new renditions of their hit songs. The singers were backed by a band led by disc jockey Joey Au Sun-ming and background vocals provided by Albert Liu and Brenda Lo. New singers like Faye Wong, Softhard Core Kids, Andy Hui, Tony Leung Chiu-wai and Cass Pang did admirably although they were a bit nervous. Meanwhile, old-timers like George Lam, Sally Yeh and Roman Tam took it all in their stride for a very relaxed performance. Tam was a surprise hit with the young crowd, getting the most applause for his perky performance of Persian Cat .