Can you leave a Class?

[Tor]

I just tried to join the Mages Guild, and was informed that I cannot because of my belonging to other guilds.
I started as a Fighter, and have the quest to join the Celestial Order.
The Celestial Order icon in the class screen is greyed out, as we have not gotten around to completing it.

So is it possible to dropout of the Celestial Order, and switch to Mage guild? The NPC stated if you dont complete the quest you can never join the guild. As far as I can tell though, I can't "drop" the quest either.

Thanks all!

[fable]

No.

[Frodo1]

Fables right, you must either stick with these selections, go back to an earlier saved game, or start over.