Wirelessly posted (SAMSUNG-SGH-I617/UCIB1 Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile 7.11) UP.Link/6.3.1.20.06.3.1.20.0)
There is a way to have one mileage number for benefits and another for accumulation, but your best bet is to just change it at the gate after you have your boarding passes