Continue using the free VPN client. If you need support, the Fortinet Community Forums are your best resource for troubleshooting.
Contact a Fortinet partner to purchase FortiClient EMS licenses . This will allow you to "onboard" your existing free clients into a managed environment, effectively upgrading them to the full version. Continue using the free VPN client
While you cannot "upgrade to a full free version" for technical support, the jump to a licensed model is what turns FortiClient from a basic utility into a comprehensive security solution. If security and support are your priorities, the licensed Endpoint Management Server (EMS) is the path you need to take. Continue using the free VPN client