Will this mean that you can't join a realm if there are no vacant estates?
I can see this causing a reason for expansion, which is good, but it may also cause lords to create nice estates to lure in new knights, then changing them to make room for the next 'bait estate'. If this is implemented (I do like the idea!) we should make it harder to resize estates... maybe a maximum that you can change an estate/lord's share per day?