Did you downgrade to 4.2? I'd say that's a bit risky... You probably could restore it with Gateway as long as it's a 9.2 image from your 3DS, but messing with the NAND makes me nervous. It would be safer for you to find a game with 6.1 or 8.1 or something and update using that. But then if your 3DS was on the internet long enough, it could have downloaded the update files for 9.4, which it will use instead of whatever's on the cart; if it did that, you'd be absolutely SoL. Not sure what I should tell you to do, to be honest. I downgraded my old, beat up 3DS that I just call my "guinea pig" lol. I use that for my xorpads or whatever 4.2-specific stuff I need, and all my games and such are on my 9.2 3DS.
If your 3DS wasn't on the internet though, I'd say it would be safe to use Bravely Default to update. That'll get you on 6.2.
Just be careful.
Yeah I downgraded to 4.2 so I could use the ds profile exploit method and use the blue cart for ds Roms. I currently have a 9.4 emunand with eshop and everything working. I was totally cool with losing all my saves then remembered I had 40+ hours on Bravely Default and have yet to finish. I have already once successfully restored my nand while it was still on 9.2, but like you said doing it on a downgraded nand makes me nervous. If there's nothing I can do but risk it I might just bite the bullet and restart BD some day.