If you used styles wisely when you created the document--for instance, assigning the Heading 2 style to the subheadings and the Normal style to everything else--you've got it covered. The job is easy:
Go to one of the subheadings--it doesn't matter which one. Select the entire paragraph, and change the formatting in whatever way you wish (or your boss wishes). Then right-click the paragraph, select Styles, then Update style name to Match Selection. For instance, if your subheadings use the Heading 2 style, you'd select Styles, then Update Heading 2 to Match Selection.
But if you didn't use styles to begin with, things will be more difficult. (And you'll probably use styles from now on.) You need to replace the formatting that currently defines subheadings with a style. You can do this with Find and Replace.
With the document up, press CTRL-h for the Find and Replace dialog box. Leave both the 'Find what' and 'Replace with' fields blank. If there's a More button in the lower left corner, click it.
Click the Format pull-down menu button. Select the options for the formatting that currently distinguish the headers. For instance, with our bold-and-underlined example, select Format, Font. For Font style, select Bold. For the Underline style, select the appropriate option.
With the cursor in the 'Replace with' field, click Format, then Style. Select the appropriate style, such as Heading 2.
Click Replace All.
Now that your subheaders have their own unique style, you can use the instructions described above to change them--and only them.
When you're processing words, you shouldn't have to worry about things like formatting and protecting your files. You should be free to concentrate on the really difficult parts of the job, like coming up with a good closing paragraph.