PowerPoint guru Steve Rindsberg, in his PPTools website (http://www.rdpslides.com/pptools/) has a great post on how to do this here.

If they have used masters you can go into the master (from the main menu go to View -> Master -> Slide Master) and change the fonts in the text boxes and titles there. If the user has stuck to using the master then all of the slides will be updated. - Updated Feb 8, 2006