I got everything ready today, updated to 10.3, installed homebrew channel, placed update files on sd, etc, etc. Followed all the steps up to when you start sysupdater. I'm getting the black screen when launching. Probably a "god send message" to tell me to stop now and wait for a better release. I can't find 1 good, reliable source, confirmation that they got n3ds working properly with downgrade. Those people that claim success later post about questions about why things that should work, don't work, etc. And consensus looks that if you somehow get the updater to hobble through and downgrade, you won't be happy with the results. Not on a N3DS that is.
See
https://gbatemp.net/threads/people-with-n3ds-n3dsxl-sysupdater-read-this-before-downgrading.408292/