This is a new fee that BA is rolling out world-wide. You can avoid it by using another Oneworld airline to reissue your ticket (in HKG I would use CX, which does not charge this fee). The fee is not related to the OWE fare and probably didn't exist when you bought your ticket (but the fine print allows imposing such fees, it seems). BA will waive this service fee if you are a good enough customer of BA, or just avoid using BA for the re-issue (while any OW airline can reissue any OWE ticket, they are reluctant to do so unless they are flying one of the trans-ocean segments).