Can I reassign AA as my credited program after flying a BA flight I earned Avios on?
 

I took a BA flight in March, on a BA ticket. I originally used my BA Avios number on my reservation. Can I ask BA to remove/cancel the earn from that flight, and then submit to AA for mileage credit?

You can ask

You can ask but they are almost certainly going to say no. I personally wouldn't spend the time trying to ask given the tiny probability of success. Once miles have credited to one program it's incredibly hard to switch to another one, even in cases where the airlines have made an error.

It is possible

You would need to ask BA to reverse the credit in your BAEC account and then retro claim via AA and the process can and does go wrong. It's always better to use the right FF number in the first place.

And BA may be as reluctant as AA apparently are in refusing such reversals if you have used any of your status benefits such as BA seat selection or lounge access.