You can downgrade it with signed firmware but it looks like Bitmain always remove older version from their download page.
Maybe some user here who use the same miner has a backup or older firmware version but I don't recommend to download them from unknown source unless if they have a bitmain direct link of older version.
if you use a non signed firmware ( like AsicTuner overclock ), it wont work on NiceHash ( maybe not compatible with their "Asicboost" ? ). But if you use original firmware, even the latest, it works perfectly.
To downgare, just recover 1st firmware ( not on their website, they removed it ), put on SD and flash it through the Sd reader.