While it is possible to unschedule a scheduled post, it is sort of a work around. It would be nice, no, it would be excellent(!) if there was a small link underneath the "Update Post" button that said "Unschedule". Unscheduling would do two things: 1) remove any date next to "Publish on" and 2) set the post to "Draft". A simple click is all is needed instead of changing the date to the past, etc, etc.