Yes, I had 30K images on S3 and I needed to move them to other external storage provider. Ended up doing it all manually and I still have that Amazon S3 placeholder in my external storage settings.
What I did was moved all the images from S3 and created a new external storage in the settings with the same url. Good thing I used the custom subdomain to serve images from S3, so it was much easier. I am too lazy to write few lines of db query to migrate to new external storage. Why bother, everything is working just fine. 🙄