I can open sysupdater, but it crashes regardless if I press a to upgrade or y to downgrade.
Not exactly sure where you currently are in the process with the crashes. (Assuming half-downgraded between 10.2 -> 9.2 and stuck)
I have the exact N3DS (20th Pokemon anniversary) and went through similar issues.
By now you should probably try updating to 10.3.
Look for the 10.3 CIAs and replace them in the update folder on the SD card.
Try to boot up sysupdater and update to 10.3. (May require multiple tries as sysupdater always crashed on me)
Once you get 10.3, delete the update folder and replace it with the 9.2 CIAs. Then use browserhax to get into homebrew and install menuhax.
Once installed, replace the boot.3dsx on the root of the SDcard with the sysupdater.3dsx
Now try using menuhax to directly boot into sysupdater and repeat trying to downgrade. (Took me almost 50 tries since it would just freeze)
(I found that browserhax didn't work so well trying to get sysupdater downgrade working, so I used menuhax as an alternative and it worked out for me)
-
Personally, I only went to from 10.2 -> 10.3 then -> 9.2 because I somehow softbricked my O3DS downgrading with safesysupdater from 9.9 directly to 9.2. (It required 10.3 for safesysupdater downgrade?)
Just did it on my N3DS as a precaution to avoid softbricking again.