No answers - but I've had a similar experience recently.
Redemption Club tickes booked MAN-LHR-SYD (as no LHR-SYD reward seats available at time of booking). Had called back to add the return and asked the agent to check if LHR-SYD was available for the outbound, advised that it was but first leg on the BA11 rather than all the way through on the BA15.
She claimed that there were two additional charges to make the change, first was the same as above - higher tax on the ticket (despite removal of the MAN-LHR sector) PLUS additional Singapore airport taxes because we were now 'connecting' from the 11 to the 15 rather than simply stopping off on the 15.
Refused the change as the total quoted was more than the train to Manchester and night in a hotel...
Cheers,
TheEscapist