There will be break in I-94 continuity as the old one expires in Sep and the new one cannot start until Oct. So it would be better for L-1 employer to file for your L-1 extension. This way, you can continue to stay on the basis of pending L-1 extension even after Sep 1. Otherwise, my suggestion would be to leave US and enter on stamped H-1 visa.
I would say the best course of action for you would be:
–>Travel out of USA and back during the time your L1 visa is valid. At the port of entry the I94 that would be handed out to you is generally greater than the visa expiry date.
This way even though ur visa would expire on the 1st of september your I94 validity would enable you to stay on for a month.