I did a reissue of my DONE4 in BKK ealier this year.

I tried Cathay first, but they didn't want to do the reissue because it was a BA ticket. They send me to the BA/Qantas office which did in fact do the reissue for me.

BA made a big mess of my ticket (they replaced coupons of my paper ticket with etickets), but at least the reissue was cheap. (No extra fee, and the saved taxes were subtraced from the 125$)