Solution This cannot be done by the user side. Log a new ticket to the support team or call to the support center according tohttps://www.fortinet.com/support/contact.htmlto get release on the old DDNS record. After the release the DDNS record, register the record using the same domain on the new unit.
To use your old FortiGuard DDNS with the new FortiGate firewall device, you will need to log into your Fortinet account and make sure that the hostname and authentication token are still valid and associated with your new firewall's WAN interface.
After you have confirmed that your FortiGuard DDNS settings are correct, you can configure your new FortiGate firewall to use the same hostname and authentication token that you used with the old firewall. This will allow your new firewall to automatically update the IP address associated with the hostname whenever your ISP changes your public IP address.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.