Or like Ark where you can just overlay the new structure on top and once it's completed it's upgraded. You have to have the full materials for the new structure but it's more realistic. I like the upgrading a grass wall into a sturdy grass wall with the repair tool once you've added a weed stem but I would also like to be able to completely change the material to another (grass into weed, weed into mushroom - also need a mushroom floor because that would be awesome imo).