Not seen this discussed yet but I thought it was worth bringing up.
As we all know, if you have the standard version of say Wrath of the Lich King from back when it came out, and then decide to upgrade it to the collectors edition for the goodies you can do so and freely obtain those rewards.
Flash forward to Warlords of Draenor. Would it be safe to assume that if you first bought the standard edition of the game got one free 90 and then upgraded to the collectors/deluxe edition that you would then receive a second free instant 90? Technically you are buying a second copy of the game and placing it over the standard one. All without the faff of having to transfer the second 90 between other accounts and so on.
Just an interesting thought for those who don't mind paying extra for more alts. Wouldn't benefit from it myself, got 11 characters on my realm.