1.  Stop application on the primary.
2.  Wait for the sync to be completed and up to date. 
3.  Stop replication.
4.  Upgrade primary nodes.

(After the upgrade on primary completes, VCS will start the application. We need to remove the entry from autostart list for the application group to avoid application start)
    
5.  Upgrade diskgroup on primary
6.  Upgrade secondary
7.  Upgrade diskgroup on secondary
8.  Start replication with -f option
9.  Upgrade disk layout version
10. Start application

Note: Primary and secondary can be upgraded at the same time.

In case of any data differences observed between the sites after the upgrade, full sync between primary and secondary sites needs to be performed. 

Infoscale 7.4.x VVR/CVR upgrade limitations due to the enhancement in the SRL write format.
Cause In Infoscale 7.4 release, SRL 4k aligned write feature was introduced to optimize the SRL writes to improve the VVR performance. Prior to this release, SRL writes were 512 aligned, this caused a performance drop on faster SSD devices like NVME. So, this new enhancement was brought in to boost the VVR performance. As a result of the above change, the replication can't be performed across the nodes which have different version/release ( i.e is between 512 and 4k aligned SRL writes) as that could lead to data corruption. When it comes to VVR usually phased or rolling upgrades are performed, however, it isn't possible when upgrading from 7.3.1 or earlier version to 7.4 or later versions as the SRL write aligned are different in 7.3.X and 7.4.X. So, the upgrade must be carried out after ensuring replication is up-to-date and completely the replication is stopped. Start the replication back only after completion of upgrade on both primary and secondary cluster nodes.