My exact procedure was as follows, all on an old 3DS XL (games and NNID were installed, I did not unlink the NNID before the procedure):
Made a full SD card backup
Downgraded from 10.3 to 9.2
Used emunand9 to dump the sysNAND
Used emunand9 to format the emuNAND partition on my SD card using...