When the start date of a series is changed, the exceptions from the series (like when deleting an occurrence) are moved along with the series. This is not always the expected behavior, because in some cases the exceptions should stay in place (because they are holidays for example). It is also an issue because the users don't understand why some exceptions have disappeared, or why other exceptions have appeared.
I would suggest that the end user be presented with an option to: keep the exceptions in place, move the exceptions or delete the exceptions (like Outlook does) when he changes the start date.
If the exceptions should be handled the same way always, the developer could specify the handling way, and then the user would not be alerted.