Troubleshooting Tip: Forward Error Correction cannot be changed

The FEC must match on both sides of the link for an interface to come up. However, FortiGate does not allow changing the forward-error-correction of an interface that is a member of an LACP group.
When changing the FEC option on an interface, the FortiGate will return an error.

The following is an example with port 40 having FEC set to cl91-rs-fec, featuring the command to disable it:

config system interface
    edit port40
        set forward-error-correction disable
        show
            config system interface
                edit "port40"
                    set vdom "root"
                    set type physical
                    set mediatype cr4
                    set snmp-index 44
                    set forward-error-correction cl91-rs-fec
                    set speed 100Gfull
                next
            end
        set forward-error-correction disable
    next
Interface speed cannot be changed when there's aggregated interface in same group.

The error above mentions interface speed, but it is actually referring to Forward Error Correction.
The command will be accepted, and the configuration will show FEC as disabled:

show system interface port40 | grep forward-error-correction
    set forward-error-correction disable

However, the setting will not take place, and checking the interface with the following command will show that the interface is still using CL91 for Forward Error Correction:

diagnose hardware deviceinfo nic port40 | grep FEC
FEC :CL91 (0x3)

This is expected behavior, as the error mentioned that this FEC setting cannot be changed while the interface is a member of an aggregate (LACP) interface.
In order to actually change the FEC, the interface will need to be removed as a member of the LACP group first.