Over on the EU forums is an ongoing discussion of a type that usually comes about every expansion cycle: the question of whether or not your computer will need upgrades in order to run the new content. The short answer from CM Taepsilum is that, if you're happy with your computer's current performance in WoW, you're probably fine for Warlords of Draenor. So there's the answer to that.
However, fleetingly mentioned at the beginning of the post is the following interesting tidbit:
This isn't something that's been explicitly clarified before now. There's been some talk that there would be a way to switch between the old and new models but we hadn't heard exactly how that would be done. It seems we now know: just set your graphics to "low" and it will enable the old player character models. If you've been nervous that your beloved main of many years might be changing irreversibly, hopefully this allows you to breathe a little easier. There'll be a way back!