Complex sentence: Until 2014, they divided their purchases between Mercedes-Benz and Volvo, but in 2015 they commencing the transition to a fleet of 450 Mercedes-Benz cars exclusively.
Split into simpler sentences: Until 2014, they divided their purchases between Mercedes-Benz and Volvo. In 2015 the company changed its purchasing policy, commencing the transition to a fleet of 450 Mercedes-Benz cars exclusively.