OP, thanks thanks thanks. I had to create an account just to thank you for this.
Ok, so I just successfully downgraded my US n3ds (on 10.3 firmware, it was the latest available) first try without issues. I was a nervous wreck, didn't have a NAND backup, and was worried about bricking.
I know there are a lot of questions out there, misinformation, incomplete tutorials, etc. So I basically did what was in this video, and it worked beautifully.
1. I have a 32gb PNY MicroSD card. I backed up every single thing on my n3ds card to my PC. I used the SD Card formater that Nintendo recommends
right here. I selected Full (Overwrite) Format Type and Format Size Adjustment ON. Then I formatted the microSD card. When finished, I copied everything right back over from my PC to my newly formatted microSD card. THis included all my emulators, roms, and everything else I had installed via Browserhack.
2. I didn't Format my n3ds or anything, just the microSD card.
3. Then, I followed everything in the video in the first post in this thread, exactly as stated in the video. I used Menuhax as my entry to launch sysUpdater. But before launching sysUpdater, I did the FTPBrony trick by opening it, then immediately spamming the B Button to close it. THis is also detailed in the video. Also, if you had a DNS set up to use Tubehax in the past, you need to remove that before doing this downgrade. I never used Tubehax so this wasn't an issue for me.
4. I used the sysUpdater that is in the description of this video. I know some people have had issues with different sysUpdater programs.
4. Everything worked just like it did in the video.