Hi Dialungana,
In essence the upgrade is relatively simple for swapping out the appliances. Depending on the new hardware and the difference between the interfaces etc it could be summarised as the below.
1) apply offline base configuration on new appliance with correct interface IPs and routing along with the rest of the Gaia configuration.
2) During a maintenance window turn off old appliance and bring online the new one. (Could be done by disabling and enabling ports on your switch)
3) Update the gateway object within SmartConsole, version, reset SIC. Make sure your interface topology is correct within SmartConsole to what has been configured within Gaia.
4) Install Policy.
And you should be good to go. This assumes that you have a dedicated management server and that is running R80.20 and you wish to keep the same gateway name and IP addresses.
Just out if interest which appliances are you migrating from and to?
Regards
Mark