Yes, trying to downgrade to 4.1.1, assuming you get past the Odin bootloader check, could hard brick the phone.
I have unlocked three phones (all of them were i747m ) using the free method but none of them had been upgraded to 4.3 so we'll have to wait for someone to try using the free unlock method on a phone that is running a 4.1.1 ROM with a 4.3 bootloader.