For his third album, the son of Richard and Linda Thompson lights out for country territory, covering hits made famous by everyone from George Jones to Elvis Presley. He leads off Upfront & Down Low with languidly gorgeous versions of Boudleaux and Felice Bryant's ''Change of Heart'' and Dolly Parton's ''My Blue Tears,'' and his thoughtful tenor reaches into the very soul of Merle Haggard's ''The Worst Is Yet to Come.'' Throughout, Thompson makes gloominess seem like a state of exalted grace.