We ensure Region-->Zone-->Node hierarchy is always maintained. We do not allow region assignment without zone. 
oo-admin-ctl-district -c set-region -i <server-identity> -r <region-name> -z <zone-name>
oo-admin-ctl-district -c unset-region -i <server-identity>
For changing zone, they should unset the current region/zone and set the current/different region/zone.