What maker are you using? It makes a difference here because 2k3 has a change class event. But if you're using XP or 2k, then you've made classes yourself, and there is no change class command, so it will have to be done slightly different.
Nevermind, I'll just assume you're using 2k3. Make a common event, set it to parallel process. Make a conditional branch, where the condition is whatever the Hero's Level is for when you want his class to change. Make sure the custom handler box IS NOT checked. Then, inside the branch, just make a change class command. Do this for every hero.