http://www.aa.com/i18n/aboutUs/arriving.jsp#faqs
if the combination of your Million Miler balances allows you to reach a new million mile level when the programs combine you will receive the benefits of that level. Any benefits you would have received for prior million mile levels will not be awarded. For example, if the combination of your million mile balances allows you to reach three million miles, you will receive AAdvantage Platinum status for the life of the program along with four systemwide upgrades. Any benefits you would have received for achieving one million and two million miles in the Dividend Miles program will not be awarded.