I have just uploaded a 'Copy Event' plug-in to the
Unofficial CalendarScript - Mods and Plugins site which will allow copying of a current event's details to either the same calendar or a different one. Once selected, the user just enters the new schedule and which calendar it is to be posted to. The original event stays where it is and is unmodified in any way.
It may conflict with some other plug-ins if you're using them so read the installation instructions for details.
Dan O.