It's the first time I hear about this, but it doesn't make sense. From
Wiki:
In the announcement, Modi declared that use of all ₹500 and ₹1000 banknotes of the Mahatma Gandhi Series would be invalid past midnight, and announced the issuance of new ₹500 and ₹2000 banknotes of the Mahatma Gandhi New Series in exchange for the old banknotes.
If they could just exchange old banknotes for new ones, how is that killing cash?
Wiki also shows there were certain limits per person, which can easily be solved by some more black market trades: sell ₹4000 old money for ₹3950 new money.
After the exchange is completed, all illegal activities can just continue with new banknotes.