Nano changed to PoW means the zero fee for all transactions failed.
Must have some cost for tx, fees,computation or other form.
According to Medium article (linked above) "Pow significantly increases the cost of flooding the network with spam transactions,
without effecting the average user."
The team explained the POW in details how it works so it is good to read this to understand about the recent changes. I hope this will put positive impact on the growth of this coin.