If your document is saved in docx or docm format you can change its filename to end with .zip and then open the document with a zip editor. The images will all be stored in the word\media folder and these can be replaced/updated in the zip.

However, if you are changing image formats then perhaps the filenames there will need to be changed to reflect those changes and so perhaps then other xml files in that zip will also need to be modified to reflect the new filename and/or file format.

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Andrew Lockton
Chrysalis Design, Melbourne Australia