I am fairly certain there is no way to import existing tunnels into VPN Manager. So yes you need to create new tunnels and effectively start fresh. But honestly when using VPN Manager it's probably easier to just create new ones anyway because you have to do the work to define the communities and gateways regardless.
Once the new tunnels are pushed out to the existing FortiGates you can use routing protocols or SD-WAN to ensure a smooth migration over to the new tunnels. And then when everything is confirmed OK just delete the old tunnels.
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