Originally Posted by James Sanders
... could I just sign into the BA site using my booking reference number and select an option to upgrade using Avios?
Welcome to Flyertalk,
James Sanders.
I've never tried to upgrade a BA sector that is part of an xONEx but, in general, upgrading any BA flight that is part of a multi-sector itinerary -- even xxx-LHR-yyy, with both segments on BA -- cannot be done online; it requires a call to the service centre. Good luck.
cheers,
Henry