This is a good lesson that you should be using paragraph styles. If you create and apply paragraph styles for the Header and Body Copy, all you have to do is change the definition in the style, and the work is done.
You can maybe use the Find/Change
See if that can work for you.
But like Steve said it is good practice to use Paragraph Style whenever you have lots of Type, I just get lazy sometimes.. lol
Find/Change, nah! Use Find Font, from the Type menu.