US WPEN radio station (AM 950) switched formats from adult standards to oldies today, the Philadelphia station's devoted listeners - many of them 75 and up - took the change in stride. People like Emma Black of Clementon, who is old enough to remember the pre-Sinatra days. Told the 75-year-old station would now favor Elvis Presley over Frank Sinatra, she said, "I love Elvis Presley, believe it or not. When he sings `Are You Lonesome Tonight?' I just melt, because I've been a widow for 40 years." As devoted as listeners were to the station's mix of big band, Sinatra and pop standards, it wasn't enough. In the spring Arbitron ratings book, it was 24th out of 25 stations in the Philadelphia market, with a meager 3.0 share.