Call BA or use the chat feature and ask them to change it. But remember that Avios can be very useful in the US for internal flights on AA and Alaska, and One World partners internationally. I find it very useful to have a stash of Avios and AA miles because sometimes there are better options with one over another. With that said as a US resident there are more opportunities to acquire BA miles than AA miles (after you have exhausted the Citi and Barclays credit card bonuses). Also as a BA silver/One World Sapphire (or higher) I think you get access to domestic AA lounges which you won’t if you qualify for One World Sapphire via AA.