You can search some discussions about config "conversion" or "migration" or similar keyword through this forum. But basically two options: Converter or manual conversion.
There is a piece of software called FortiConverter. I don't know if free trial is available or you need to buy a license.
If you want to go manual method, you almost have to use CLI and need to have good knowledge about all components of configuration inside the config file, at least what you're using, like
"config system global"
"config system interface"
"config firewall policy"
"config vpn ipsec phase1-interface"
"config vpn ipsec phase2-interface"
"config webfilter profile"
and so on and on.
Then you would need to know what need to get copied or what part needs to be modified before getting copied. For example, interface names need to match the new HW interface names obviously. And as long as interface names are ok with the new HW, you can copy all policies as long as you have copied all protection profiles. You need to know all those conditions. Once you have a good plan, the rest would be: copy from the config file, modify, then paste into the new FGT.
If you go that path, I would recommend you first upgrade the previous FGT to the same FortiOS version, or at least the same major version, like 6.4, 7.0, which you are intending to deploy with the new FGT, so that at least all those config components are the same.
Toshi
