In a statement, general director William Mason said that Smith's voice had evolved to a point where it was no longer appropriate for the leading role of Mark, the groom in the eponymous wedding.
"[Music director] Sir Andrew [Davis] and I engaged Mr. Smith more than four years ago after his very successful Lyric appearances as Laca in Jenufa and Foresto in Attila in our 2000-01 season," Mason said. "He came fully prepared and is singing wonderfully, but over the last four years his voice has grown in size and heft and darkened in color. We mutually agreed that the role of Mark requires a different vocal aesthetic and have agreed to release him from his contract."
The Midsummer Marriage is making its Lyric debut, in a staging that marks Tippett's centenary. The production is directed by Peter Hall and conducted by Davis; it runs November 19-December 16.
Kaiser, a member of the Lyric's training program, appeared with the company last season in Aida and Fidelio. He sang the leading role of Mosca in the world premiere of John Musto's Volpone at Wolf Trap Opera in March 2004 and appeared in Baz Luhrmann's Broadway production of La bohme. He was recently selected to play Tamino in Kenneth Branagh's film of The Magic Flute.