You can place a new house, it will start your old home decaying. You will have 5 days to move all your stuff to the new house. Then drop the old one, and wait 2 days for your account to reset. Then you can replace in the old spot. The only problem is anyone will be free to place in that area. But with so much open land I dont think anyone will place a house, unless it's in a prime spot like Luna. I know of 3 or 4 spots right now for keeps and more then 50 for 18x18's.
I always enjoy a little risk so what I would do is test the size you plan on upgrading to, wait untill an odd hour, drop the house and replace it. Then drag all the back stuff inside, I have did this a few times and never lost an item. The key is making sure you know what will fit. You can also load all the good items in 5 packys.
Goodluck with what ever you do.
Tom