Every class as an availability is the current thinking, but is not necessarily the final thinking on this. We shall see what happens. Also, you will not be transforming one class into it, you will be able to unlock the class for play. We felt that it was important for players to not feel as if they had to give up a long term character for the chance to play the new heroic class.

There is a perception that what you will be doing is choosing a specialty class that would be an extension of your current class, but that's not how we're doing it. We felt that if we did that for all of the classes, we'd be adding far too many classes at once and making continuing efforts toward class balance very difficult.

Via WoW Forums