Tango,

Twice I have had similar situations such as yours. I had purchased a non-refundable ticket from LAX to LHR, and also from LAX to Canada. I had to cancel my flights, but since it was non-refundable, I couldn't get my money back. I had to go to the local CTO (it is definately to your advantage to develop a good relationship with your local CTO for times like this ) where they sorted out the situation.

Because of the fare class, for the unused LAX to LHR ticket, I was only able to apply the value of the ticket to a higher priced fare class, and the value was only good for one year from the date of purchase. I had to pay a $75 change fee.

For the LAX to Canada, for a $75 change fee, I was able to apply the unused ticket's value towards any other ticket. Any remaining would be returned to me in the form of a voucher. Please note, as with any cash value voucher, the voucher can be used for any ticket(does not have to be your ticket). I was then able to use this voucher to purchase a ticket for my wife.

A key item to note about the taxes is that the voucher is for funds, TAXES ALREADY PAID. The CTO agent was very good and she caught onto this (a little extra work on her part to make it all work). For my wife's ticket, we did not have to pay any additional excise taxes, and in fact her ticket was cheaper than mine (for the exact same itenerary!) because of the lack of excise taxes.

To make my long story short, how you can use your unused ticket depends entirely upon the fare class in which it was booked. Also, don't let the airline charge you for excise taxes twice on the same funds.