If you are trying to reformat an existing document so that it uses the formatting and content of another template, do the following: Create a new document based on the "other" template. Then use the Insert File dialog box to bring in the existing content from your existing document.

To display the Insert File dialog box, click the arrow on the Object button on the Insert tab, and then choose Text from File (note that the command is misleading, because all content, not just text, will be inserted).

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional