I could be completely wrong on this.  But from what I have read on the world of warcraft website... you have to have at least one character that is lvl 55 or higher to make a DK.  When you make a DK, it is not the character you already have that "becomes" a DK.  You are in fact Creating a NEW character that Starts at lvl 55 for the DK class.

Has anyone else taken the wow site description to mean this as well?