generally when you get into PhD program, you will get assistantship as well.  you do not want to enroll PhD without having an assistantship.  so, i think you are pretty safe to go back to nepal and come here as a phd student.  i believe there is no certain time frame like you have this many days to leave USA after your H1b expires unlike F1 visa, which allows you grace period of 60 days.  however, certain time after H1b cenceling is understandable i guess.  this can be explained....