It's not a question of whether the image of Demi Moore on the cover of W magazine has been altered, it's a question of how much. Covers of glossy magazines will almost always be retouched, especially those featuring celebrities, for reasons of vanity on their part and also to maintain the perfect ideal of the publication in question.
Depending on the desired outcome, this can be minimal and simply involve colour balancing, or it can go to the extreme of removing blemishes or reshaping features.
On fashion shoots, it's common for the model's hands to go blue if it is cold, and sometimes it's quicker to put colour back into the skin in post-production. Sometimes a shoot will be naturally lit and will hardly be altered in post-production, but photographers aiming for a more dramatic result will often add special effects, working on an image so heavily that it can become unrecognisable from the original.
The incredibly smooth and even texture of Moore's skin indicates that the photo has been digitally enhanced. However, it seems ridiculous that W magazine would need to superimpose a model's body on to the original photo of Moore – after all, they are working with the best photographers and stylists in the business and their subject is a beautiful actress.