+1 - This is the risk and the very reason why BA and other carriers make the distinction between changes in advance and changes while away from point of origin. Depending on risk tolerance, e.g., what happens if "my new ticket costs 5x what I could have paid" or, "I'm fine spending an extra week in JNB"

For what it is worth, AA's new software to weed out "impossible" bookings would now prevent (or result in cancellation of a new booking). Whether that is AA's, is proliferated for use by BA or is OW-wide, is something I don't know.