You will not be transforming or converting one class into a Death Knight, you will instead be able to unlock the class for play. The current thinking behind this design decision is that it is important for players to feel like that they don't have to give up a long term character for the chance to play the new heroic class.

Many people had the perception that what you would be doing was to choose a class to specialize into the hero class that would then 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 would be adding too many classes into the game at once and as a result making continuing efforts toward class balance very difficult.