I am not sure what consumer right was violated here (as per American law) as I don’t know the American system, but I would have thought basic contractual law is in play here. They offered to amend your original ticket to an open ticket valid till 2021 end, in exchange for not having a refund. You accepted. They then renegaded on that. You should be able to claim for the cost of the new ticket as a minimum through a small claims court with demonstration of TK’s agreement (public statements to that effect being an example).

You may also be able to file a chargeback via the bank for double billing of the same service (again you would need to demonstrate your PNR confirmation and TKs agreement to it).