Suppose it comes down to what amount of work we can expect. Many of the trees for instance are just models pasted onto the base zone, so updating those could instantly given the illusion of the zones being updated. Same with changing the skybox or going over the grass texture used in all the zones and swapping them for a higher resolution one.
There are definitely ways to update the zones in a way that does not require going into each zone to change it, but we will see. Cataclysm had to make all zones useable for flying, so the question really is what is the large amount of work, remaking assets into higher resolution, or changing zone layout drastically.