To copy styles into a document from a template that you have just attached to the document, you MUST check the "Automatically update document styles" option.

Unchecking the option shouldn't trigger an error message, unless Word can't find the template for some reason. However, you can always attach the Normal template instead. It won't change the style definitions, as long as "Automatically update..." remains deselected.

An alternative approach, at any time when you want to reformat a document, would be the following: Create a new document based on your template. Bring in existing content via the Insert File dialog box. (To display the dialog box, click Insert > Object > Text from File.)

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional