If you submit your PPE pricing between September 15-30, it will have the standard 14-day notice period for your existing users. Between October 1 to the date your new pricing kicks in, you will not be able to get any new rental customers, but you will continue to receive the pro-rated revenue from existing rental customers.
When your new pricing kicks in, you will see the deductions on your invoice that reflect the refunds your rental customers have received on their last-paid full month of rentals (you'll see the full amounts they paid as the income for your Actor during that month with the adjustments appearing as a lower line in your invoice). If their last day is September 30th or October 14th, this migration will be treated the exact same, the only difference is that you can't get new rental customers from October 1 onwards.
This does mean that, if you're playing the zombie game and waiting until the last possible moment to make your change (to juice zombie rentals), you can continue to bring in that revenue until 14 October at the latest but you sacrifice new customers for the interim.