I understand your frustration, but I believe what NW charged you is correct. Typically, once a coupon is used, it's used, and they're not going to give you an exchange credit for more than you actually paid.

Essentially, NW adjusted your original fare by $75 at the time of ticketing to lower your cost basis. So when you go to refund or exchange that ticket, you have the new lower fare to apply.

Similarly, say you were to purchase a $100 item at a department store, and used a 20% off coupon, for a total spend of $80. If you decide next week you no longer want the item, you wouldn't expect that store to give you a $100 credit because the item was "worth" $100 when you bought it. You paid $80, you get $80 back, but the coupon is gone since it's been used. It's that simple. Better luck next time.