Hello fine folks of the BA forum!

I'm doing YYZ-JFK-LHR-BCN in 2 weeks, arriving into LHR on AA 100 (in J), and departing on BA 480 @ appx 1300. I booked this connection to give sufficient buffer in case of delays, but looking back at it, and given the high liklihood that I'll be stuck in the confines of T3, I'd rather try and go standby on one of the 2 earlier flights to BCN (BA 474 or 478).

What is BA's policy on moving up to earlier flights? Is there a fee? Can I do this in the T3 Flounge, or should I try doing this at the gate for the earlier flights?

I'm AA EXP, for what it's worth.

Thanks!