I have found relevant community posts under InDesign scripting. This will take some homework, but I consider this post closed at this point.
First of all, I hope you work with Paragraph and Character Styles.
Go to Find font, every font in the document listet dan be replaced by any font and you can redefine your paragraph styles.
If you have build up your paragraph styles in an intelligent way you can change the font in the root style, all other depending on it will inherit these changes.
I can see this is the right way to think about it rather than scripting.
The last thing I did a few years ago was to start adapting these documents to accommodate styles.
If you have properly implemented styles, and they are named identically through all the files, you can add the files to a Book (.indb), edit the styles in one document, then synchronize across them all.
If not, then frankly scripting may really be a better option.
Hi. Did you try..
Create a Book (file/new/book) and insert these documents into a book and synchronize from style?
I found the font in my old Microsoft Office 2008, this is a clever near term fix. (I can only work this on weekends.)
I have a long term interest in maintaining these documents, which do fit into a self authored book, and deserve fields, styles and book format.
Many thanks to all contributors to this discussion, especially friends in Europe, for prompt and relevant responses.