I have a similar question, might as well ask here.
Trip IAD-LHR-DUB in last week of August thru first week of September. Work, but also a mini TP-run so I can keep Gold (November year end, so no extension).

I would like to change the DUB to LHR flight, but MMB keeps giving me the desired DUB to LHR plane (BA 831) but insists on changing the LHR-IAD leg to LHR-BWI.
I can't seem to get the "Change your booking" to do what I want, as going to BWI is out of the question. Trying to be sneaky and selecting the BA831/BA229 hoping I could do a separate transaction to move to IAD got rejected. It's too risky to keep trying that as I really don't want that flight.

Full information about the booking: Paid with cash plus Avios. Upgraded to First for the IAD-LHR leg with cash. (Part of my evil plan to get enough TPs.)

The BA Gold line strongly admonishes you about calling for flights that aren't coming up soon, but I really do want to change this so I can sync arrival with a family member (and bring them through the First Wing).

So, ignore the warnings and get on the phone? Or is there a better way to get booking changes to give a bit more flexibility?