I'm having issues with getting this to auto boot on a dsi with HiyaCFW. Autoboot.bin is in the correct folder as well as having the launchargs in the dsiware folder. It's enabled in the CFW settings as well as having it enabled in DSiMenu++ (If it's enabled the option should show "Restore DSi...
That just means the exploit failed to gain access to the service calls it needed. Shut off your 3DS and try again. It hasn't touched the NAND so don't worry about it.
I just attempted a downgrade on a New 3DS XL, Firmware was 10.3.0-28 EUR.
Before I attempted it I moved my DSi titles to the SD Card and then erased them from the SysNAND thinking they might interfere.
Then using Menuhax2.1 I started sysupdater (Julian's Libkhax Build) the first time it failed...