I have found changes to a BA rtw ticket trivial, if you are not in a rush, ever since I found they have a rtw department you can email. It takes them a working day or two to assign someone to a query but thereafter responses are quick and phone calls are possible.

(Yes I will have the possibility of higher taxes to contend with of course but for ex cai as mine was or ex sez as it sounds you have there is limited choice)