At least one person should warn that downgrading is always connected with a high risk of losing parts of the config, as stated in the Release Notes:
Downgrading to previous firmware versions
Downgrading to previous firmware versions results in configuration loss on all models.
Only the following settings are retained:
interface IP/management IP
static route table
admin user account
system access profiles
Background: firmware updates contain the new firmware and possibly script code to change previous syntax to the new version. Sometimes, the internal HDD filesystem is reformatted. If you downgrade just by applying the older firmware, the wrong routines are run (those for upgrading) and this might result in loss of function.
So, for a remote FGT, I'd be very, very cautious. Perhaps only the IPsec VPN might be broken afterwards which in this situation would be quite bad. This way or the other, downgrading is risky and needs extra effort. As you may have noted, several others have downgraded just by applying the older firmware, and it worked for them.
I take back my statement this version is running fine.
IT'S A DISASTER! Both units we were testing in production have had serious issues. I will be rolling them back to 5.2.8 tonight, hopefully that goes as planned. But 5.2.9 is a BUGGY MESS. Stick with 5.2.8 if you are on it, and wait this one out.
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.