After Long time got a chance to working with Fortigate, almost 8 years i guess.
Took some time to understand old 310B and with 3 days all commands are same as expected. some syntax changed.
Old Legacy 310B with HA Active/ Standby having issue with frequent deamon crashed and it was old version of code which was not supported by fortinet any more.
So now time to replace with new kit.
Steps to follow high level :
Since old config is not compatable with 6.X FortiOS
Offline with VM configuration ported from 5.X and upgraded to VM as per the path to 6.X
Take the backup config from VM and import the configuration to new Device 501E.
once Primary configured and just configured Secondary Management Ip and connect to HA port so the configuration will be replicated atuomatically.
High level steps.
run the new kit parallel to old kit. remove the old kit cables and connect to new kit ( make sure clear the ARP entries in Switch)
you expect to have small downtime like 5-10min for the new kit come online and serve the traffic as expected.
it was easy migration never done this closest small downtime window with any other kits.