There would be absolutely nothing wrong with Square Enix retroactively removing multiple plot ownership from previous rulesets. Give the individuals or FCs that own multiple plots 30-days to decide what they want to keep so the others can be vacated or auto demolished. Implement a calamity salvager function for housing plot items so nothing is lost either. If there's no response, just auto demolish based from newest to oldest until one plot is left. SE should even encourage users to be proactive about this up rewarding free game time as that at least offsets some of the negative PR.