Why do you want to convert AA miles to BA? Except for transatlantic flights, you can redeem AA miles for BA flights (and vice versa). And you'd lose about half the mileage in an AA->DC->BA transfer. And BA tends to need more more miles for the same flights in some cases than AA does. And with BA you often pay higher fuel surcharges than with AA. So for all these reasons, it is often much more valuable to keep the miles as AA miles, even if you want to fly BA or other OW members only, than to transfer to BA.