I saw that article and asking here because of the "To deploy this HA, you will not launch FortiGate and other related resources from marketplace product listings. Instead, you manually kick off deployment using ARM templates." Which means we'll have to deploy a new cluster using ARM templates and move all the ipsec tunnels. I had some bad experience in the past migrating tunnels and just wondering what to expect here. Not sure what to expect in this case as we have also Azure there.
Converting a standalone Fortigate into a HA manually can be quite tricky in Cloud (especially in Azure), as it would require adding more NICs/Subnet and a SDN Connector in order to failover the PIP to the new node (plus you may need a bigger instance with 4vCPU in order to have 4 NICs in place).
You may could try to follow the steps manually as described in our Github:
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.