It's not retroactive, it just prevents Player A from controlling to much. Player A can no longer have a potential access to 24 airships after this change.

While Player A might have a Player B that has a FC house if they were to ever leave or even move I think it would disable their access to the house since the house would be refreshing itself as the moment of change.

The tricky thing becomes how does SE control it? I imagine it might be like Crafter Souls where you have the ability to change 3 times a week but for FC housing it might be you can only swap once a week. This would at least give players option to change their focus if they want to be at FC house over other house without being exploited since they would still only be limited to access to one airship/fc craft.