Alrgiht. This gave me a bit of hell, but I got it working.
NEW3DS I was on 9.9
I used this tool. It siad all of my files were good.
I downgraded (had to rety about 7 times)
Everything acted fine, and I had to manually reboot.
Turned it on, "Error Has Occured" bullshit, with and without SD card:
Went into recovers (Up , L , R , A , Power)
Updated to 10.3
Used browserhax to install 10.3 themehax
Worked first try to downgrade
Had to manually reboot,
Error has occured.
Did recovery again,
Took out SD card,
Formatted ( LOST ALL MY STREETPASS STUFF

)
Used browserhax,
Downgraded first try
Manual reboot,
Booted Fine, Checked settings 9.2.0-20u
Ran @ihavemac 's tool and said I successfully downgraded
Going to install CFW in the morning
Thanks for releasing this tool. Helped so much

Did I mention I lost all my streetpass data??
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So I want to use this on the new pokemon 20th anniversary N3DS, but i probably will have below 10.3 firmware, is it mandatory to have 10.3 for this? because i think 10.4 will be out before i get my N3DS so i dont think i'll be able to get 10.3, just 9.x/10.x whatever the N3DS comes with. is that okay?
Any firmware 9.3-10.3 will work.