I believe that would screw up a lot of players. Customizable Castle Lots would go for more than 800 Million. They already go for 500M!
This is what I'd do.. Customizable Lots take up a square area. I'd allow players to do ANYTHING they can imagine with this area. Completely open the area up and create L-shaped houses, two towers connected with bridges, 4 small houses with no ground underneath(meaning the Lot's ever-persisting floor we cannot remove).
And of course allow people to lock anything down any way they want, floor or no floor(within that area).
I hope I make sense here. Anyhow I'd allow this change first, then maybe move on to allowing Customizable Castle Lots to avoid the boom in price increase. It would be balanced by the price increase in ALL Lots then.