Class question

[Munchkiny]

First of all, I already read the FAQ section that explains when each card can or cannot be played, I'm not trying to harass anyone with my noobness. But my question has to do with a specific munchkiny deed I was not allowed to perform during a combat and it's been haunting me because I think the move was legal based on what I have been reading in the forums so far:

I was a Thief and engaged in combat during my turn, not as a helper or anything, with one monster. I was a long way from being able to kill it. Before trying to run away, I realized that one of the two cards I had in my hand was a Wizard class card. So I discarded the Thief class card and put the Wizard class on the table; then I stated that I was using my remaining card to charm the monster (the monster would have given 5 treasure cards at that point if charmed).

One of the other Munchkins argued that I could not change class during combat, that in combat I could either discard the class I already had in play, or put a class card in play if I was without class or had a card that allowed an additional class.
Is this accurate?

The thing is that in the end, I had to take the Bad Stuff from that monster. But I'm guessing now that I should have been able to pull off what I wanted to do in the first place.

Thanks for all your help.