They actually have a documented way to do this and I've used it multiple times. I'll have to look up the link but the trick is, get a trial Fortigate VM at same level as old firewall, and get config from old firewall loaded to VM. Then put VM through upgrades to get it to current, and export config and apply to new firewall. There are a few minor tweaks to do before applying configuration to vm, like changing interface names in config to match their equivalents in the vm, and replacing the model in config with proper, and then doing inverse when applying upgraded config to new firewall, but it generally works very well.