To make the situation a bit more clear and to clarify.
We cannot really stop anyone from modifying the game in such a manner if they so wanted when it comes to simple drag, drop, replace cases. The thing I want to put some weight on here however is that by no means should anyone alter how the meshes actually works when it comes to the collisions and other potential matters how they will appear for others in the game and it's official maps. Re-texturing and other mesh swapping is A-Okay if so wanted as long as it's plainly visual but anything that changes beyond that we cannot say will be allowed.
Changing visuals is fine, how an actual mesh works not so much.