I used the script provided by @solverat, because it was advised. There were some issues with the script that I have tried to address, see the bitbucket site.
There is one issue that is not easily solved with the documented ACF hooks:
The script currently processes image fields by fetching the thumbnail HTML. However, the alt attribute of this image tag is not updated (and why should it?), and the alt text is stored via a separate AJAX request. Is there any way to fetch the image details via AJAX?
