This article contains the information how to upgrade SRX1500 and SRX4k devices in cluster prior to 20.4 using minimal downtime method.

To perform Junos upgrade using Minimal downtime procedure, minimum Junos release required for SRX1500 is 20.4R1 and for SRX4100/4200/4600 it is 21.1R1 .
Prior to these release the below commands will not work. User require to manually plug out the control link.
set chassis cluster control-interface node 0 disable
set chassis cluster control-interface node 1 disable 

All the steps described in the below documents are same except disabling the control link command. Please refer this document below along with this knowledge article for details.
Minimal_Downtime_Upgrade_Mid (SRX1500-SRX4k, Junos 21.1 and higher)

Here are the steps to perform the Minimal downtime upgrade for SRX1500 and SRX4k chassis cluster.

The below steps assume that Node0 is the primary for control plane (RG0) and data plane (RG1+) and configured with a higher priority than the secondary node. As needed, please failover all redundancy-groups (RGs) to primary node
>request chassis cluster failover redundancy-group [x] node 0

Step 1 : Disable all interfaces on node1

Step 2 : Disable Syn and Sequence check 

Step 3 : Deactivate preempt if enabled already for all RG's 

Step 4 : Deactivate interface monitoring / IP monitoring for all RG's:

Step 5 : Commit the config:

Step 6 : Disable fab link:

Step 7 : Manually plug out dedicated control link
Step 8 : Upgrade node1

Step 9: Verify you are able to commit the config on both nodes. If yes, then proceed to disable interfaces on node0 and enabling interfaces on node1. 

Step 10: Verify traffic is passing through Node1 

Step 11: Upgrade node0

Step 12: Connect control link on Node 1 and re-enable Fab link on node1 only

Step 13: Reboot node0

Step 14: While reboot starts on node0, connect control link on node0

Step 15: Verify the below on node0:
Note: It may take few minutes for node0 to detect node1, where node0 will be in hold state.
- Both nodes showing same version

- All FPCs and PICs are online (may take upto 15 minutes depending on the type and number of FPCs)
- Chassis cluster status should reflect nodes as Primary/Secondary

- Chassis cluster statistics should reflect increasing counts on control and fabric links

Step 16: Enable all physical interfaces for transit traffic on node0, Interface / IP-monitoring and enable TCP syn-check/ sequence-check.
Step 17 : Verify the below:
show chassis cluster status show security flow session summary
show chassis alarms
show system alarms
show system core-dumps