Welcome to FlyerTalk, brown249.

AA upgrades on domestic flights only work for AA Gold and above, not for BA Silver or above. Ie, just because AA and BA are partners doesn't mean they extend all the benefits of their program to their partners, only a limited subset of benefits.

So if you're trying to earn BA status, you have to use your BA frequent flyer number for your AA flight, and you won't get access to any AA upgrades.

However, once you get to BA Gold (oneworld Sapphire), you might be able to reserve exit row and/or other extra-legroom ("MCE") seats if you wish at no cost. If so, that is the greatest extent that you can "upgrade" on AA using BA status. But I'm not absolutely sure about that; I'm just extrapolating from the fact that on some oneworld carriers my Sapphire status (from AA Platinum) has given me the ability to reserve exit row seats that "mere mortals" can't.