The level 10 class quest from each guild unlocks the ability to go to the other guilds and obtain their "So you want to be a <best class ever>" quests, which in turn unlocks the ability to switch to <best class ever>.

Go to one of the other guilds and talk to the receptionist. Take the quest. They'll jabber at you for a bit about the class. Take the quest again and tell them you really do want to be a <best class ever>. They'll send you to the guild master. The guild master will jabber some more, then ask you if you're really sure. Say yes again, and you'll get a starter weapon for <best class ever>. Equip it and you are now a <best class ever>. Talk to the guild master again for the <best class ever> level 1 class quest.