Here is the fare rule pertinent to OP: (my underlining for emphasis)

AFTER DEPARTURE CANCELLATIONS PERMITTED. NOTE - --CALCULATE DIFFERENCE BETWEEN FARE PAID AND QUALIFYING FARE FOR TRAVEL COMPLETE USING FARES OF ORIGINAL TICKET DATE / REFUND RESIDUAL VALUE IF ANY TO ORIGINAL FORM OF PAYMENT.

Basically, your stopover point becomes your destination, and your original ticket value will be compared to the oneway ticket to your new destination. If the new price is lower than the original, then you are due a refund to the original FOP. If the price is higher, chances are that nothing will happen, and the unused flight coupon will simply be forfeited.