Sorry guys i could not explain you very well in my last posting. In order you to be in F-1 status, you must have valid I-20 and INS has nothing to do with VISA stamp in your passport and it is only for the purpose of leaving from Nepal. There should not be a gap of more than 60 days between the date your first I-20 expires and the date that your new I-20 gets effective from. If you can do this, you are okay no matter whether you start from summer or fall. If it seems like that there may be a gap of more than 60 days period between two I-20s, you may be out of status for that certain period. In that case, it is advisable to go to Nepal to avoid that out of staus situation. Even you don't have to re-apply for new visa from US consulate in Kathmandu if your visa stamp is still valid.