Costello was forced to retired because learned that Genovese soon or later would kill him; Stanfa was the official boss and stepped down only because get life and NY give the role to Skinny Joey.
As I know Frank Morelli was the first boss of the Providence crime family from 1917 to 1932 when he stepped down becoming underboss to Buccola.
Giuseppe "the Clutch Hand" Morello in 1922 stepped down becoming underboss to Masseria.
But was in the pre - commission era. In the other cases or the boss was killed or force to retire (if lucky).