This article explains how to transfer a FortiGate configuration file to a new FortiGate unit of a different model.
1.  Open the backup configuration file from the previous and different FortiGate Unit.
2.  Download a backup of a new configuration file from the new unit. This procedure is different depending on which FortiOS version is running on the FortiGate:
In FortiOS 3.0, 4.0 and 4.1.x, download a factory default configuration file from  System > Maintenance > Backup&Restore
In FortiOS 4.2, 5.0 and 5.2 download a factory default configuration file from System>Dashboard > System Information > System Configuration

In FortiOS 5.4 download from Dashboard > System Information > System Configuration > Backup or Admin > Backup Configuration.

In FortiOS 5.6 download from Admin > Configuration > Backup.
3.  From the factory default configuration file copy the "config-version", and paste this value and replace in the backup of the previous configuration file.
Make sure that all interface names correspond to the new device.  For example, the previous unit may have had a "wan1" interface however the new device has a "port1" interface, it is critical to make sure these correspond.
Save the new configuration file under a new .conf file. This step is mandatory otherwise when reloading the new configuration file the error message "configuration file error" will be displayed on the web based interface.
Only copy the "config-version" section of the first line of the config file from the device being copied. In this way, upon conversion to the new device, the correct "vdom" and "opmode" settings will be applied.
4.  Verify which user admin account was used when saving the configuration file. Reloading a configuration that was saved under a super_admin account to a simple admin account will display the error message "invalid username or password on the web based interface.
5.  On the new FortiGate unit, go to System > Status, select Restore, and upload the edited config file to the new unit.  The unit restarts automatically.
6.  Test the configuration.
It must be noted that modifying .conf files in this manner will not ensure that all profiles will be saved. This is particularly true if this procedure is used for .conf files being used on a different versions of FortiOS. For example, reloading a .conf file to a FortiGate running FortiOS 4.1 from a .conf file using FortiOS 4.2, any new profiles related to new FortiOS features will be lost.

Fortinet Support for the transfer of a configuration file

Transferring a configuration file from one model to another is not supported by Fortinet, however part of the configuration can be restored manually by copying the required configuration from the old backup configuration file to new configuration file.

The Fortinet Technical Support department does not offer technical assistance in converting FortiGate configuration files from one model to another as, when required, this is the responsibility of the user.

Refer also to the SysAdmins Notebook article 'Transferring a configuration file from one model to another'

Customers can also choose to purchase the FortiConverter tool if they would like to transfer files from intra platform. Eg Cisco to Fortinet.

The FortiConverter tool is available if a configuration is to be tranferred between platforms, for example from Cisco to Fortinet.