Type Return (to accept the name of the copy in the title bar--My Document Copy, for example)
Type Command-` (to switch to the original file's window)
Type Command-W (to close the original window)
Press Button 'Revert Changes' (to revert the original document to the point of the last save)
Yes, it's a $36 solution to what shouldn't have been a problem in the first place, but it works with any application that supports Mountain Lion's new Save As implementation.