can we use iRules to dynamically change the resource pool in a VIP

You could use a DataGroup to do this and have all your iRules reference it. You could then make it as simple or complex as you need it, however, simplistically it would look something like

ltm data-group internal /Common/environment
records {
32bit { }
}
type string
}


    when HTTP_REQUEST {

        if { [match class 64bit equals environment] } {
         pool MachinePool_64
        } else {
         pool MachinePool_32
        }
}