Problem of "Magic Overlay"

Hello, I am currently creating a magic skill system from overlay. And I want to know how to make it so that when I press a certain key, it will switch to the next overlay. The problem is that the next overlay may change if I learn a new magic, and I can't make the new overlay show up. Please help me.

Note: If you feel confused, I will describe this problem in more detail.

Sorry for my bad English.