the materials for units are not bound to maps they are a consent over the entire game only way to change the texture is to change the material / material instant constant (MIC) texture that is used on the unit (if you change the package you can not join a server with out it re-downloading that package and reverting the change you made)
the only way textures can be changed is if the client and server have the same package (witch is a good thing to stop cheaty textures)
but to change the texture in the MIC code needs to be added ether in game or via a mutator to swap out the def, spec, normal, ect texture's a bit like we do with the cubemaps and colour of units in the map world props