Yes but for BA long-haul - unlike AA - they don't offer same day flight change - it's the same rule for everyone and no OneWorld or BA status is going to change the rules on paying fare difference, if you didn't buy the more expensive flexible fare to start with

Bad luck, sorry.

Also goes without saying but obviously as a work-around DO NOT book a separate "short-haul" from XXX to LHR and then expect to pick up your LHR-SEA connection - if you skip the first short-haul leg on your connecting ticket the whole itinerary will be cancelled.