All done. KWI it is.

As a result of the debacle that ensued with my ticket over a six week period earlier this year following my involuntary re-route (which involved me wrongly being denied boarding in LAX earlier this month because AA believed I hadn't paid the UK taxes), AA waived the change fee. As pandaperth suggested, the taxes were the same for LHR-KWI as they were LHR-JED. Because of this waiver I regrettably can't offer anything more definitive on whether taxes are reassessed on only the changed sector or on the entirety of the remaining itinerary. In that respect all I have is noted above and commensurate with what Calchas notes: taxes are reassessed on all remaining flights.

Thank you all for your help with this one. It's all worked out in the end.

Also a bug-bear of mine when people seek assistance but don't report back.