It is in the rules of your fare. A non-refundable fare is always non-refundable unless you meet the provisions of the fare to make it refundable if it is even there. The provisions, if they exsist, are usally for death or serious illness. Most fares will state if the fare has been upgraded, that the non-refundable portion of the fare will always remain non-refundable. Also, if one were to mix fares on a ticket, the most restrictive set of rules would apply.