My experience with CX is quite mixed - sometimes they charge extra taxes (mainly if adding UK departures since that is the biggest tax by far, or if adding CX sectors where they can easily calculate the extra fuel tax), but most often they don't, and on an occasion where it was clear that the taxes were reducing significantly they wrote off the change fee. But that is as a CX Diamond doing my reissues at HKG airport a couple of hours before I need to take the next flight on the ticket.