You would have to be hardmodded and make a nand backup via that to get a nand backup if you are on 9.3+
I believe we need more than just ARM11 kernel to have full access to the nand via software.
So... Get the downgrade file when it gets released, then hope and pray.
Or if you want to be extra careful then get a hardmod done and backup nand with it before doing it.
Also nand backups from other 3DSes would only work on that specific 3DS.