Thanks for your replies.

Is there a Oneworld document that stipulates handling of taxes specifically, in terms of rebooking xONEx (I could not find anything specific on the Oneworld Explorer PDF on the website) or is this actually up to the issuing carrier/TA to determine?

I've only done *A RTW's before (first DONE4) and I can't recall taxes for unused flight segments not beeing offset with new flight segment taxes. So I was interested in hearing if this is a OW xONEx thing, but from the responses so far, it seems not.

I would not have a problem with service fees etc as this is mentioned in the rules (and of course if the taxes were actually higher for the new flight segments). But when I asked about the charge AY tells me taxes for previously charged flights are non-refundable/void and will not be taken into account when calculating taxes for the new flight segments.

Cheers,
Andy