It doesn't do us any good to tell us what names you've given your classes... it doesn't matter if you call them soldier, cleric, and hero, or huey, dewey, and louie... those are just names, they mean nothing to the code.

The important question is, what the flying #$$! is a class? Do you have things like mob/cleric, mob/hero, and mob/dewey? Or just one general mob with a class variable that can be set to contain the text "cleric", "curly", or "hero"?

In that case, having an NPC that can change your class freely is problematic... the only way to do it is to create a whole new mob and switch the player's connection to it. I'd reconsider either your class structure, or the class-changing thing.