No they can't just put what is best as they are tied in by contract agreements. That is why Red Bull tossed Renault aside as soon as the contract expired. They had problems with reliability and have gone to Honda now. They used to be with Honda ages ago and Honda pulled out saying they couldn't justify the spend when they were making drastic spending cuts around the world. It will be interesting to see which manufacturer comes in to supply engines in the future besides the 4 that are currently the only ones.