downgrading should be fine (if you're not sure about this, try formatting the SD and/or use another SD card); if it fails in the middle, just continue and DON'T UPDATE BACK TO FIX unless you can't access home menu at all
.......though there's reports of Old3DS having it happen but chances of it are very low. seems to be dependent on what firmware the console came with, which is like 9.4.
formatting the console makes it safer for some reason. also you wouldn't lose eShop purchases, but would friend list. I'm pretty sure you won't brick; if it does, you can update and get a hard mod
vb_encryption_vb said he doesn't mess with normal o3ds models though. So I'm out of luck for a hard mod. I'm just gonna hope it doesn't brick, when I do downgrade, that is.