Assuming anyone gave enough ***k about a DSi to nand mod one, they'd still need a dump from before they updated. Like with the 3DS, you can't downgrade unless you have a backup of the old sysnand from before the update.
Though the DSi's security isn't as good as the 3DS's was, so the firmware encryption might have been cracked/exploited. But I don't think anyone ever really bothered with it.
