@Julie3802 Whenever you change a setting, unless some tech glitch occurs, it only applies to new bookings. It shouldn't affect already confirmed bookings. You could decide to change your check-in window, for instance, but the confirmed bookings are governed by the check-in times that were in place when that guest booked.
BTW, that prep time setting is very confusingly worded. If you set one day prep time, it will block one day between reservations, which is how it should be worded. The blocked day after a reservation doubles up as the blocked day before the next available date that can be booked.