From the rules
16. VOLUNTARY CHANGES / REROUTING / PENALTIES
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(c) Upgrading
1. An Economy Class oneworld Explorer fare may be upgraded to a higher tiered or First/Business Class oneworld Explorer fare without fee.
2. A Business class oneworld Explorer fare may be upgraded to a higher tiered or First Class oneworld Explorer fare without fee.
3. A First Class oneworld Explorer fare may be upgraded to a higher tiered oneworld Explorer fare without fee.
4. Upgrading by sector to Business/First Class is not permitted.
5. At any time the oneworld 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 --NON REF/ XXX-- (XXX equals oneworld 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.