How a new CEO could make a mark on the company.While Starbucks got a lot of positive press following the hiring of its new CEO, Brian Niccol, Chipotle Mexican Grill (CMG -2.78%) just lost the individual who had helped guide it successfully over the past six years. Let's look at the potential effect on the company and whether the stock is still a buy.CEO jumps ship to StarbucksBrian Niccol jumping ship to go from CEO of Chipotle to taking the reins at Starbucks shocked the investment world, sending shares of ...