Since Styles are at the heart of how Word works, you'd be well advised to learn how to use them - including how to modify the existing ones and how to make your own. See:
Understanding Styles in Microsoft Word - A Tutorial in the Intermediate Users Guide to Microsoft Word
How to Use Microsoft Word Styles for Document Formatting
Apply styles
Customize or create new styles

Styles can also be copied between documents using the Organiser.

See: Using the Organizer to Manage Styles (Microsoft Word)

Making them take effect in a document that uses an earlier definition of the same Styles is as simple as checking the 'Automatically update document Styles' option, under Developer|Document Template.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]