But JAF does have a downgrade option. I guess it is safe to downgrade if you have JAF box and FBUS cables with you.
it's safe to downgrade when you check the platforms. some phones allow you down via usb with jaf. generally old s40 like 5300, 6300, 3120c etc. from smartphones i've tried e51 - sometimes ok, sometimes not. but at least you can fix it by normal flash.