What's going to also be a pain for them to do is make the BC areas of Azeroth actually exist in the same instance... right now if you go to the draenei or belf areas you actually go into the same instance as Outland... the ghostlands and eversong are not physically connected to the Eastern kingdoms right now... not only this but the continents are stacked vertically on top of eachother hence why if you fall through the world in Northrend you can end up in the EK or Kalimdor dead... also if you look at a full world texture map you will see that the ocean around Northrend and the old world are different colors...
Also there are the outdoor world versions of some instances like Zul'gurub, Zul'farrak and then the whole southern half of Silithus... all of those things need to be overhauled... and of course the several empty and unnamed zones such as the northern plaguelands the areas to the west and east of Burning steppes... there are loads of unused portions of landscape in each zone as well... will they just leave them as mountains?
All in all it's going to take a huge effort to bring the old world up to par with northrend... or even outland for that matter although outland still carries some of the vanilla wow flaws...