@Deb3707 Anything you change about your listing only affects bookings made after the change. So already booked reservations will be under the previous Flexible policy.
I don't know that there is a "most commonly used policy". It depends on what a host decides works best for them. A host in a location where guests tend to book months ahead may use a strict policy to avoid guests' bookings blocking their calendars for months, only to cancel a few days beforehand, leaving the host with empty daates they have little chance of rebooking.
Flexible may work fine for hosts who get the majority of their bookings last minute, because they live near a major airport or interstate and usually will have another booking roll in right away.
I have always used Moderate, and it has worked well for me.