A large organization uses Palo Alto Networks VM-Series firewalls deployed across multiple availability zones in Microsoft Azure. These are managed by an Azure Virtual Machine Scale Set (VMSS) and integrated with an Azure Load Balancer for high availability (HA) traffic inspection within a Transit VNet.
The security team needs to perform a critical PAN-OS software upgrade across the entire fleet of firewalls with the requirement of minimal application downtime.
Following Palo Alto Networks best practices for highly available cloud deployments, what is the recommended approach for safely performing this software upgrade with the least downtime?