Well, Word applies the Caption style to a caption as you create it. When you modify that style, the modifications should reflect on the formatting of existing captions as well.

The exception is if you have applied direct formatting (for example via the buttons on the Formatting toolbar) to your captions. You would then have to select the captions and press Ctrl+Q (resets paragraph formatting to that of the underlying style) and Ctrl+SpaceBar (resets font formatting to that of the underlying style).

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional