The node ID value ranges from 1 to 255 and is used to identify each device in the chassis cluster. The cluster ID is also used to identify each device, but it is not part of the node ID configuration. A system reboot is not required to activate changes to the cluster, but it is recommended to ensure that all changes are applied properly.

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.
Reference:
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.

In order to manually failover the primary Routing Engine in an SRX Series high availability cluster pair, you must issue the command "set chassis cluster disable reboot" on the primary node. This command will disable the cluster and then reboot the primary node, causing the secondary node to take over as the primary node. This is discussed in greater detail in the Juniper Security, Specialist (JNCIS-SEC) Study Guide (page 68).