Changing Document Formats Globally
When Word creates a new document, it copies the entire contents of the template to the new document—text, pictures, headers and footers, and so on—and then establishes a link between the document and template (so, for example, styles in the template become available in the document). With one possible exception, after a document is created, nothing from the template gets copied into the document. So, for example, if you change the Normal template so its default font is Garamond 11 point, all new blank documents will have Garamond 11 point—but all old documents based on the Normal template will stay just as they are.
Tip from 
There's one huge exception to this rule: If you choose Tools, Templates and Add-Ins, and then check the Automatically Update Document Styles box in the Templates and Add-Ins dialog box, Word "updates" (that is, wipes out) a docu ment style if one exists in the template with the same name. This option is good news if you always, without exception, want template styles to control the look of a document. But if you sometimes create styles within a document, avoid this option.
So unless the Automatically Update Document Styles box is checked, you can make all the changes you want to a template, and the documents associated with that template won't change a bit. Changes to the template affect only new documents based on the template.
How do you make global paragraph and character formatting changes to a document—that is, make global changes to the appearance of a document—without going into each style, each paragraph, and making the changes manually? Word gives you three options:
Allow Word to make changes for you, either piece-by-piece (in the AutoFormat feature) or wholesale (by using the Style Gallery). Neither of these options is likely to improve the appearance of the document.
→ To learn how you can put Word's AutoFormatting features to work for you, see "Automatic Formatting".
Change the template applied to the document, and then force Word to update all the styles in the document so they conform to the styles in the new template. You might want to do this if, for example, a coworker comes up with a spectacular new template that handles all the styles you commonly use.
Apply a theme. Because themes include background colors, and background colors rarely appear in professional documents, this approach is virtually unusable unless you're working with a Web page.
To attach a new template to a document and update the document's styles, follow these steps:
If the document has any styles defined inside it (that is, at the document level), save a copy of the document. This process can completely overwrite styles in the document, and you can't bring them back.
Check the Automatically Update Document Styles box. Click OK.
Close and reopen the document. With the Automatically Update Document Styles box checked, this flushes out any styles with identical names in the document—overwriting the document's formatting with the new template's formatting.
Caution
The Word documentation says you can leave that box checked "to ensure that your document contains up-to-date style formatting." That's true for documents based on standard corporate templates, where you absolutely, positively want the template to dictate every style choice. It's also the right choice if you fully understand the relationship between the document you're working with and the template to which it's attached, and you want to use only the styles in the template. If you're experimenting with styles in a document, however, leave this box unchecked. You'll avoid headaches caused by disappearing document styles.
Before you consider applying a theme to change the appearance of a document, make sure you understand what a theme entails. A theme contains
A background pattern (few of the themes that come with Word have solid-color backgrounds)
Style definitions for Heading 1, 2, 3, 4, 5, and 6, as well as the Normal style
A graphic for bullets
A graphic for horizontal lines
AutoText entries specifically designed for Web pages—the "created by" entry puts the originator's name on the page; "created on" inserts the date
Themes can be handy if you're trying to create a Web page that matches the formatting generated in a different program—FrontPage or PowerPoint, for example. They're also reasonably useful if you're developing a Web page from scratch. But they're effectively unusable for any other kind of document.
When you apply a theme to a document (choose Format, Theme, per Figure 18.5), Word does the following:
Figure 18.5. Apply a theme to a Word document by choosing Format, Theme, and choosing from the list.
Changes the background of the document (choose Format, Background) to match the pattern in the theme.
Replaces the document's (not the template's) style definitions for Normal and Heading 1, 2, 3, 4, 5, and 6.
Sets up default bullets and horizontal lines to match the graphic in the theme.
Puts the AutoText entries in the template (not the document).
Caution
Be careful when applying themes to existing documents. If you apply a theme to a document and then decide to remove it by setting the theme in the Themes dialog box to (No Theme), you'll find that the styles you defined in the document for Normal, Heading 1, and so on, don't work. Sadly, those styles you created are gone. There's nothing you can do.
You should expect a lot of additional anomalies if you decide to work with themes. Animated graphics appear in themes, but they won't be animated in Word—you have to use your Web browser to see them move. You can't create or modify a theme in Word. You have to use FrontPage. They don't print.
Themes are clearly a "version 1.1" addition to Word. Use them with care.