No, from the website 'How to play'... At the end of each Mini Series, you will be able to change your Jockeys and Trainers. Two changes are available in between each Mini Series making eight changes in total during the game I think the first Mini Series ends at the end of May (last race 31st May), but I'm not 100% certain, just guessing as I haven't seen a definitive list. I'll have another look later...