Technically, Death Knights were meant to be relatively small in number, and could only have previously been paladins. Which kinda means only Blood Elves, Humans, Dwarves and Draenei would be DK races. I guess it kinda worked like Star Wars' Jedi/Sith thing. You didn't just immediately become force sensitive to become a Sith.

They won't have any problem implementing this if they walked all over previously said lore in the first place.