Changing destination - through flight on same aircraft
I am not used to changing flight bookings, but I know I’ll likely need to phone BA / BAEC on this and I would welcome BA FTer’s advice on the following so I know what to request of the agent when I call.
I did a booking and upgrade for myself and Mrs fish to GRU for next autumn, WT+ MFU’d to CW (unfortunately there was only MFU availability for GRU, not EZE). Out on BA247 and back on BA246. However, we wish to travel through to & from EZE on these same flights.
One option, presumably, would be to buy a separate WT or WT+ return GRU-EZE-GRU on ba.com, but is there a better option?
For example, is it possible to change the paid WT+ booking to LHR-EZE-LHR, given this is the same flight number and keep the MFU’d CW sectors LHR-GRU-LHR? And pay the £90 change fee plus the difference in WT+ fare (and service fees?). This would mean WT+ for the GRU-EZE-GRU sectors which is just fine.
Any other suggestions?
Sorry if this seems a basic question, but I can’t find a definitive answer elsewhere as to whether a “change” can include the destination on the same flight number and/or if that impacts negatively on the MFU.