This isn't about real life logic? But it is, they are re-using some models and setups to make sure to maintain the looks of it, or increase the speed of preperation. They had a whole segment explained on why the re-use some things. Dalaran at the topic, isn't a cut corner, for it isn't really that much of change due to well it hasn't changed through the story other than moving around and changing leader ship, and just now it's getting updated and more atmosphere.
Every single MMO I have played in my life (which is about 23 MMO's) have run with older models and art to follow up new content.