O'Brien to Take Over for Leno in 5 Years NEW YORK - Jay Leno's takeover as host of NBC's "Tonight" show in 1992 was fraught with drama and bad feelings, but he's assured the next transition will be as smooth as his nightly sign-off: "Stay tuned for Conan."   The comedian and NBC on Monday chose the 50th anniversary of the first "Tonight" show to set a special date for the 55th - when Leno will step down and Conan O'Brien becomes the fifth host of the television institution.    Announcing a transition in five years is odd for any business, let alone one that frequently plans from day-to-day, but NBC was anxious not to lose O'Brien...