Traffic: changing time when editing an instance still leaves an extra instance in the original time

User report from Comrad: "(when I) try to go in to edit an instance (rather than editing the series) - if I go in and change the timing of an instance, it will create a new instance at that new time but leave the old instance at the old time, so then I have to go in and delete that instance at the old time. If I change the timing of an instance, then it should automatically delete that old instance don't you think??"

This probably means that the replace_event_date value is not getting set when editing an instance's time.

Trace the instance-edit flow for changing an event time and inspect where replace_event_date is set or consumed. Reproduce the reported edit rather than series, then verify that moving the instance removes it from the original time while retaining it at the new time.