Originally Posted by nwa.com Contract of Carriage

NONREFUNDABLE TICKETS
1) IF, AFTER A NONREFUNDABLE TICKET/NONREFUNDABLE UNTICKETED
PREPAID TICKET ADVICE (PTA) HAS BEEN ISSUED AND BEFORE ANY
PORTION HAS BEEN USED, A DECREASE IN THE FARES OR CHARGES
APPLICABLE TO THE TRANSPORTATION COVERED BY THE TICKET (SAME ORIGIN, DESTINATION, AND STOPOVER POINT (S) BECOMES EFFECTIVE
OR A NEW FARE FOR WHICH THE PASSENGER CAN QUALIFY BECOMES EFFECTIVE THE PASSENGER WILL RECEIVE THE DIFFERENCE IN FARES
IN THE FORM OF AN MCO/TRAVEL CREDIT VOUCHER AFTER SUBTRACTING A
USD50.00 ADMINISTRATIVE SERVICE FEE.
ONCE A DECREASE IN THE FARES/CHARGES AND/OR THE ADDITION OF A NEW FARE HAVE BECOME EFFECTIVE, THE ORIGINAL TICKET MUST BE REISSUED AND ALL CONDITIONS OF THE DECREASED FARES/CHARGES OF THE NEW FARE MUST BE MET INCLUDING ADVANCE RESERVATION/BOOKING CODE/TICKETING REQUIREMENT.