If you mean you applied for H1B with new I20, your sevis should be automaticaly terminated by Oct 1. No need to worry, you are authorized to work from Oct 1.

Now the visa stamping thing. Think about yourself, if you did your schooling without semster break, average or good GPA, working in the same or similar field, chance of visa stamping is very high unless they find your name matching with their criminal list. If your name matches with that, they may make you wait for 2 to 4 weeks. Average time is 2 weeks. If you have a degree in Biology and you are working in IT, still you dont need to worry. But I have seen more problems in such cases than earlier one. This is the main reason people go to Canada or Mexico to get their visa stamped. These are things I have seen here and there. Last thing, there is no reason to get worried, rejecting H1B visa is very very slim. Only fery few people with bad luck have to wait sometime 2-4 weeks. This is just my practical immigrant knowledge. Good luck.