I have a (cheap business class) ticket purchased on aa.com - fare rules don’t allow free changes.

The routing is ARN (Stockholm) - LHR tonight on a BA flight and then American Airlines to Chicago tomorrow morning. I’m booked on the last flight today to Heathrow but would love to switch to an earlier flight. Seats are available. Asked at the airport at check in desk, they couldn’t help. Sent me to ticketing counter. They couldn’t help and told me to call AA. They couldn’t help without doing a very expensive upfare. Called BA Gold line, they tried but couldn’t help, but advised I might have a chance at the gate for the earlier flight.

Looking for advice if you think I have a chance at the gate. Would love to avoid being at the airport for the earlier flight if there’s no way this is going to work.

I understand this probably does not fall under either BA’s not AA’s same day change / standby rules as this is an AA ticket with a flight operated by BA, but looking for experience trying something similar, either positive or negative.

Thanks for any comments and happy holidays!