When a security policy is deleted, the active sessions allowed by the policy will be dropped. The default behavior is that all active sessions allowed by the policy will be terminated and the traffic will no longer be forwarded. There is no way to mark the active sessions as a legacy flow or to reevaluate them by the cached rules.
According to Juniper Networks Security, Specialist (JNCIS-SEC) Study Guide, when a security policy is deleted, the active sessions allowed by that policy will be dropped. This behavior is the default behavior of the device. There is no way to mark the active sessions as a legacy flow or to re-evaluate them against cached rules. The device will terminate the active sessions and will no longer forward traffic for those sessions.