Re: Downgrade OS
If the current version added features or changed the syntax of some commands then a downgrade would most likely result in a loss of configuration.
I would do the downgrade with the USB stick method: place the old config file and the old firmware image on a FAT32 USB stick (max. 4 GB), connect it to the FGT, and enable the automatic upgrade (System>Advanced>Auto-install). This way, as the versions on the stick have precedence, first the old firmware is installed and then the old config. Reboots automatically, several times.
Ede " Kernel panic: Aiee, killing interrupt handler!"