Warren Buffett's Berkshire Hathaway announced its biggest management shake-up in decades on Monday, including a surprise exit, a retirement, and a new position. Close followers welcomed the moves, but two warned they could mark the start of a wider exodus as the legendary investor enters his final weeks as CEO after six decades in charge.Greg Abel, the head of Berkshire's non-insurance operations, is poised to take the reins from Buffett on January 1. Carolyn Dewar, a senior partner at McKinsey and coauth ...