I think since India only has notes in {5,10,50,100,500,1000} denominations there might not be enough legal tender with just {5,10,50,100} to convert all {500,10

I think since India only has notes in {5,10,50,100,500,1000} denominations there might not be enough legal tender with just {5,10,50,100} to convert all {500,1000} notes, thus by introducing a new notes (bills) which look completely different than older 500 Rs, and 2000 Rs which are completely new, they can ensure that enough notes/bills are available.