The only thing that remains the same is your character names, class names, and levels; the entire game has been deleted and remade right before your eyes. Not even David Blaine can get away by charging you $15 a month for this type of magic trick, but Blizzard has pulled it off, undoubtedly making them one of the greatest magicians of all times.
Here is a preview of part of the magic trick, http://almostgaming.com/wow/article/are-you-ready-echoes-doom-patch-3-0.
For most dps classes, especially Warlocks, the magic trick for them has been.
Now you have DPS!
Now you don't!
TA DA!