How do you make a death knight without changing the character

When you create a Death Knight in World of Warcraft, you do not change any characters, you create a new character. No characters are lost in creating a Death Knight.

i dont want to make another character i just want my name to stay the same. e.g. i have made an account called deathtodevil and its lets say paladian draeni. i wont to become a death knight draeni with still the same name and account and subscription all nasmed deathtodevil and still in my same account.