The simplest solution would really be a GM to confirm with his FC the original intent to buy an FC House, then undo the transaction he made for the personal house and put the FC house in its place.

Though I'm not sure thats really within the scope of a GMs power.

A simpler solution still would be to allow us to change a personal house into an FC House ourselves as a feature.
I might own a really nice, large house on a good plot, then get some friends to play, we make an FC, and i convert my personal House into the home for our new FC.
Wishful thinking i guess.