You can now betray at any level.
When you betray, you can only become the good/evil opposite of the class you started out as so a ranger will always become an assassin.
The one exception to this is that a druid, warden/fury can either stay the same class or flop to the other class.
Once you leave your old home city, you will move to "Haven" and live there until your betrayel is completed.
Remember that when you do betray you will lose ALL of your spell upgrades and all your new spells will be at app 1.