Downgrade question following cancellation

Heading to the US tomorrow, was supposed to be flying BA in premium economy. They’ve cancelled the flight and have no premium economy alternatives so have sent us on an indirect American Airlines flight in Economy instead…

Know it’s a long shot but planning to get to the airport early and asking if there’s any last minute space in Premium or Business (I have Silver Status). Question is… shall I head to the BA desk and ask, or ask at American Airlines? It’s a BA booking with a BA flight number, but obviously on American metal…