The ability to sell and trade furniture has been a win-win situation for the people of Ur. Why not extend such trade to upgraded wall segments?
Instead of buying wallpapers, players could pay to have certain wall segments upgraded. Prices per segment would be cheaper than current prices for full use of the wallpaper. These upgraded wall segments could then do what furniture can: be placed, picked up, bought, sold, and traded. Everyone would benefit. Subscribed players with credits could afford to try different styles if they didn't want their whole house to be one wallpaper. They could earn money by selling upgraded wall segments to other players. And non-subscribed players could buy the wall segments.
What about already purchased wallpapers? These would be converted into the new system. Each player would be given the maximum number of segments (in a 3-story, fully upgraded house) for each wall paper they had purchased.
These idea could possibly be extended to ceiling and floors.