During the claiming period allowing lateral first come first serve relocations
Allowing relocations and limiting them only to lateral moves on newly vacated houses during the claiming period allows for house shuffling into more desirable plots, and ensures that the same amount of large/medium/small plots remain available to bid on during the next bidding cycle.
We all have preferences whether we want to have a beach front plot, a secluded nature plot, a plot by the pool, or simply in the same ward with friends. Given the current housing situation with limited wards, there isn't any decent mechanism for that.
I think even if they limited to same housing area lateral moves... goblet med -> goblet med/small in the same or different ward, that would still be better than the current system if people were worried about less plots being available for more popular housing areas...