Just yesterday a friend of mine heard about keeping your good/evil class even after you do betrayal. She played Defiler to check the rumor out but in the end she was forced to become a Mystic. Did she do something wrong or something?
As far as I know, you must switch your class in order to make betrayal sequence completed. It is done on purpose I suppose. Well, you can always switch back to your original alignment, loosing almost nothing.
There are eight classes which are forced to have a special allignment (eg Conjurer/Good and Necromancer/Evil) but most classes are not fixed. http://eq2.wikia.com/wiki/Betrayal_Timeline says a lot about this
As far as i know some class are purely good or evil, Paladin and Shadowknight, Ranger and Assassin (not sure on that one), Conjurer and Necromancer, and last, Mystic and Defiler. Other class, as far as i've read, can be both alignment and thus, does not force a class switch, so a templar could go to freeport and stay templar.
On the old pvp server, you could do that. On live servers, neutral classes can betray and keep their class. Good/evil classes much switch to the class their new city approves of.