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

[seankwilliams]

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.