[MunchkinMan]

There is no "stack." The Curse's effect is immediate and must be resolved immediately, so he can't respond in any other way than a Curse-canceling card, or other card or ability that can specifically cancel a Curse or card's effects. Once you lose a Class, you lose all of its abilities. Once you gain a class, you gain all of its abilities. Unfortunately, that's no help to the Wizard-now-Thief in your scenario since he can't play new Items during combat and has no other combat abilities.