It's like the BOOT2 on wii. You can boot program through payload (if I understand) before the actual kernel is booted on 3DS. So you can unbrick a 3DS if you made a backup of the NAND, which you should have. Also, on Luma3DS, you can be on updated SysNAND and still have the signature check patch, but I don't know if it is true for GW. I THINK you still have A9LH after updating, so downgrading would be still possible if you update.