I have both a F1 and B1 visa. But my F1 visa only allow me to enter in Aug, 3, 2013, while my B1 visa is valid till next year. I want to enter US earlier to visit my girlfriend on 15 July,2013. could I use B1 to enter and then change to F1 as if i enter US using an F1 visa? Otherwise, do i have some other way to enter US earlier than Aug 3, 2013?