Even if you get the coverted PLT status with AA, the rest of the Oneworld partners may not get the update till the 'sweep' of the system happens - which varies between a few days to two weeks depending on the carrier. So even if your status may be recognised on say SABRE, it may not reflect the same on other airlines' systems and to gain entry into the lounges you need to show proof of status, as well as to present the physical card (in the case of BA).

AAdvantage might be able to provide you with a letter stating your status level but it will still be up to each partner airline whether they will let you in or give you Oneworld Sapphire benefits because your status does not show up on their system. well, ANYONE can write a letter from the airline these days...