Before downgrading from FortiOS 6.4.14 remove virtual clustering
If you are operating a FortiGate-6000 or 7000 system running FortiOS 6.4.14 with virtual clustering enabled, and decide to downgrade to FortiOS 6.0.x or earlier, you must remove all VDOMs from virtual cluster 2 and disable VDOM partitioning before performing the firmware downgrade.
If there are VDOMs in virtual cluster 2 when you perform the firmware downgrade, the FortiGate-6000 FPCs or FortiGate-7000 FIMs and FPMs may not be able to start up after the previous firmware version is installed. If this happens you may have to reset the configurations of all components to factory defaults.