You can use the Invitation Email as a save the date and email your previous guests right from your event. When you've sent your save the date, it is easy to revert it back to the default Invitation template.
Set up your event as usual with the new graphics and details.
Click Event Setup in the Event Tools menu.
Select Web & Email Templates.
Select Invitation Email from the list of templates.
Modify the template with your save the date information. Because we use HTML tables in the Invitation Template, some people choose to select all and delete the contents of the template when adding the save the date, especially if they aren't using the RSVP buttons.
Don't forget to change the Email Title and Subject line to Save the Date.
Save your template.
Once you have sent your save the date, return to the template and click the link in the lower left corner to Reset to Default.