I really don't know why WoW don't update the textures and increase the quality, it can still have the same style, the textures getting higher quality and higher res doesn't need to change the colouring or style, not sure why people think thats the case at all.
It is a huge world and would take a heck of a lot of work to do, but frankly so much of the game looks 10 years old that its a bit of a joke at this stage.
Games are about gameplay, but same gameplay and better looking is better than same gameplay and worse looking.
The reason I think it needs a whole update though is, these daft graphical improvements they've done for a couple years have essentially been crap, shadows, crappy shadows that really make very little difference or slightly improved newer area's make the game feel odd and disjointed, update everything or nothing IMHO, a very very half hearted graphics update with one new effect that looks out of place makes it look overall worse rather than better.