Join Date:Joined: Jun 2012
The stock 4.3 AT&T bootloader cannot be downgraded and, once you are on the 4.3 bootloader, you cannot use Odin to flash a stock ROM and recovery. You can install custom ROMs with a 4.3 bootloader, it's just that it can be difficult to get the phone working if you run into problems.
On a non-4.3 bootloader, you can use Odin. If you want to run a custom ROM that is not based on a Samsung ROM, you should be fine with a 4.1.2 bootloader and radio.