By GEORGE LOOMIS [NY Sun, 26 March 2008]

Just when it looked as though, after four tries, the Metropolitan Opera’s current revival of Wagner’s “Tristan und Isolde” would proceed on Tuesday evening with the originally announced cast intact, the nemesis that has dogged the run sprouted another hydra-like head. Ben Heppner, in voice and appearance the very image of good health, belatedly joined the cast as Tristan. But Deborah Voigt, the scheduled Isolde, canceled because of illness and was replaced by Janice Baird, who at an earlier performance took over for Ms. Voigt during the course of Act 2.