Comment 5 for bug 1587897

CIDR is a read-only value by design:

root@node-4:/etc/puppet/modules/neutron# neutron subnet-update admin_floating_net__subnet --cidr 172.18.161.0/25
Cannot update read-only attribute cidr

Complete subnet removing/replacing looks like a hack - it will require removal of all related artifacts (floating IPs, router gateways, ports and so on). I think it should be addressed in docs/UI