As long it is the same system and the sysnand backup is from that system then yes you should be safe from restoring it back with the backup without it bricking However nothing is 100% safe so take precautions. Just make sure you have a full charge and press up on the D pad when choosing the Downgrade option. Don't forget to back up anything on your SD card as well though not sure that matters since mine was empty.I have a sysnand backup from 9.2 and then I downgraded to 4.2. I formatted the system on 4.2, can I still restore the 9.2 backup without a brick?
As long it is the same system and the sysnand backup is from that system then yes you should be safe from restoring it back with the backup without it bricking However nothing is 100% safe so take precautions. Just make sure you have a full charge and press up on the D pad when choosing the Downgrade option. Don't forget to back up anything on your SD card as well though not sure that matters since mine was empty.
I've done this myself after testing some stuff and functions on my Gold 3DS XL; still working fine no problems.