Replace DNS LB VS to DNS Express.. when all dns server down..iRule

hi im sk. help me guys

i want to change VS(just (recursive mode)DNS LB) to DNS Express listener when all dns servers are down.

what should do for it by using irule.. is there useful command ?

here, my rule

    when DNS_REQUEST {
        if{[active_members dns_pool] < 1}{
            virtual ksk_DNSexpress_vs
            log local0. "All DNS server down! DNS Express VS start!"
        }

 }