Downgraded an O3DS from 10.3 to 9.2 without any kind of problem.
1) Just install the new version of sysupdate copying its folder into SD/3ds;
2) download from that site of iso for 3DS the correct firmware (it's easy to find the right thread. search for "browser backup")
3) unzip it and transfert its content into a folder on your root SD, like SD/updates
4) run homebrew Launcher
5) run sysupdater
6) if all is working fine right now, this one should asking if you want to downgrade your system
7) press y to accept and continue
8) OPTIONAL- praise your god, if you have one (i did it and it's worked)
9) after that it finised to install all the cia files, it should auto reboot automatically into the downgraded firmware (it seems that someone it's required to take off your SD from the 3DS to make boot properly)
10) profit
I'm not taking ANY RESPONSIBILITY from any possible brick of your console.
Please advice me if, in some way, I have broken the rules of the forum. In that case, I'll edit this post asap.