Since Borel did well with Mine That Bird in the Kentucky Derby, why is the trainer going with another jockey for the Preakness? Why not stay with a winning combination?

The trainer didn't change jockeys, the jockey changed horses. Borel is the regular rider for Rachel Alexandra and chose to go with her in the Preakness. The Derby mount was a pick up mount and he didn't have a relationship with the connections of Mine That Bird. Lots of things go into a jockey determining which horse he rides and if he has the opportunity to ride more than one horse in a race, the deciding factor could be that he wants to continue his relationship with the connections so that he gets good mounts in the future or he chooses the better horse in his opinion. Lots of betters will bet on a horse if they know a good jockey had the opportunity to ride two in one race. The logic being that the jockey wants to make money and will ride the horse he thinks will win.

Okay, thank you. I understand now.

And its not like Mine That Bird is getting a rookie jockey. His owners secured Mike Smith, who is as good as Calvin, to ride for them.