I am wondering if you start with a F2p account, then bump it up to VIP....and you buy a deluxe house, then get dropped down to Premium. Do you lose your deluxe house?
I believe that you don't actually have to be VIP to buy a deluxe house at all. The currency cap might be a problem if you're a F2P or Premium player, but if you have unlocked that I think you can buy a deluxe house even as an F2P player.
To answer your question: No, you can keep your house.