I don't think you can do that. The only thing I think you can change are transitions and title styles all at once, according to Jeff Carlson. But the individual type faces aren't covered by this. The styles applied are the Centered, Lower Third, placement styles and animation styles of the titles. Those you can change en masse by picking a different one and dragging it onto one of your existing titles. That will throw up a contextual dialog box that looks like this:

And you have many options for how to replace the style, unfortunately this doesn't extend down to the typeface, font, color, etc.