The ticket as you purchased is valid as it was at the time of purchase, as this is what you contracted for when you purchased it. The contract (the NRCoT) does not include provision for these conditions to change after purchase, so they cannot change.
The difficulty would be proving that, of course, though if sold against an itinerary it can unquestionably be used to travel on that itinerary as booked regardless of whether it should have been sold on that itinerary or not.