I've heard people claim that you can even though it's difficult, but I've never seem proof. If it's true, it's very rare. In fact, I've given my villagers walls and floors because I've never seen them display it (so it's helpful when I want to give them something but don't have anything that would look nicer in their house).
I also doubt you can get them to change wallpaper and floors. In any photo I've seen I've never seen those vary, and I've yet to visit a DA and see those changed, either. Still, if anyone HAS done it, I'd love to see the proof so I know that it can be done.
From what I know (personal experience), you can't change their flooring or wallpaper. You can only change the furniture that they display in their home. If only it were possible hah maybe it is, I don't know! It would be cool if we could although, honestly, I'd probably keep their current floors / walls