c) Upgrading
(1) An Economy class Circle Trip Explorer fare may be upgraded to a higher tiered or First/Business class Circle Trip Explorer fare without fee.
(2) A Business class Circle Trip Explorer fare may be upgraded to a higher tiered or First class Circle Trip Explorer fare without fee.
(3) A First Class Circle Trip Explorer fare may be upgraded to a higher tiered Circle Trip Explorer fare without fee.
(4) Upgrading by sector to Business/First class is not permitted.
(5) At any time the Circle Trip Explorer fare may be used as credit towards payment of any higher fare from the point of origin provided the conditions of the higher fare have been met. No fee will apply. Where such credit is towards another Explorer fare, retroactive use will be permitted. Reissued tickets must be endorsed --NONREF/ XXX-- (XXX equals Circle Trip Explorer fare basis code) and the original Non-Refundable amount remains Non-Refundable. If the upgraded ticket is subsequently cancelled, the original fee will apply.