Hello,
UPDATE: DOCXCustomizer, available in Premium licenses since phpdocx 7.0, allows to change existing styles on-the-fly.
The current version of phpdocx doesn't support changing existing styles, only create, add and import them. You can use a custom DOCX as base template with the styles you need.
The new DOCXPath (http://www.phpdocx.com/documentation/introduction/docxpath) allows to change the body content dynamically, and in the future this same class will be used to support changing other contents such as styles, headers, footers...
Regards.