ONE BY ONE: Andrew Litton, music director emeritus of the Dallas Symphony, announced his exit in 2004, but his successor, Jaap van Zweden, doesn’t officially take over until the fall. Tim Seelig announced his departure from the Turtle Creek Chorale in June 2006 — his last concert was last July. Two months after Seelig, Richard Hamburger revealed he was leaving the Dallas Theater Center — he directed his final DTC show in April; Karen Stone left the Dallas Opera last fall, she steps down from the general director post in September.