The stock is down but still trades at a premium valuation.A big shake-up just hit Chipotle Mexican Grill (CMG 0.49%). Longtime chief executive officer (CEO) Brian Niccol just announced a surprising departure to Starbucks. Niccol has been instrumental in delivering a turnaround for Chipotle after its foodborne illness outbreaks, leading the stock to 800% returns since joining in 2018, making it one of the best-performing stocks in the world over that time.Leaving a company -- especially for a competitor -- w ...