Originally Posted by
brunos
This is fairly common rule across most airlines. It is often cheaper to change the return leg after the outbound has been flown.
The BA rule posted would also suggest that historical pricing can be used before departure if the outbound remains unchanged.
There's another standard set of conditions which invokes historical pricing if the first flight is untouched. The early pandemic BUD-Hawai'i fare had that version, which was quite useful!