the way breakthrough works is that if you get +2 on a skill then you automatically unlock Y+ too and vice versa. you use the books to complete the rest when you are done. You c*so switch between Y skills and normal skills if you go and press a small *on under the skill trial section , under the vials. But you can't go a lvl down the skill that you have unlocked because you will lose all your progress. you can only go between normal skill at max lvl you unlocked or Y skill at the max lvl you unlocked