Your old I-94 will be taken at the US port. you will not have to fill out new I-94 at the port of US when you come back. Do you remember you filled out one in the airplane before entering US. Now you don't have to do that, because you already have one. You can go before October but you will not be allowed to enter USA before 10 days of your H1B validation (means before Sept 21), Like 30 days in case of F1 visa.