changed from F2to1 by mailing in 2001. Last year went to Canada on opt, visiting relatives, got F2 stamp but finally back with a warning that never go out of u.s. again, even canada or mexico. My ISO advisor then sent my stuff and got corrected to F1 on I-94. Now i got accepted to a phd program and have a new I-20. wants to go to Mexico to apply a new visa in order to go back to china during christmas. If I get rejected, can I still come back to u.s.? where can I find any related information. Thanks a lot for your kindly help.