Easily done. The easiest would be to use two separate meshes and to place both posters in the same spot, one of which is initially disabled. When the time comes, disable one and enable the other. You could parent the detailed one to the simple one and "Set State to Opposite of Parent" to make it slightly easier.
There are other more complex ways of course, such as using an animated texture for the mesh or using an overlay (transparent mesh on top of the existing mesh) if you wanted to add markings to a map for example.