Shifter now supports dedicated CDN service for medias, and it's called Shifter MediaCDN.
We want to introduce how to migrate your media on your CDN to Shifter's MediaCDN.

How to migrate to ShifterCDN from external CDN

Step 1. Stop WordPress on Shifter dashboard if you're running.

Step 2. Make sure MediaCDN is disabled on Shifter dashboard.

For more details, read Shifter Media CDN article

Step 3. Download media to your local machine.

You can media through SFTP/SSH from your media server.
If your media is on AWS S3, you can download it with AWS Command Line Interface (aws-cli), for example: 

aws s3 cp  s3://my-offload-media/wp-content/uploads  --recursive ~/backup/ 

Step 4. Start WordPress then upload media to upload directory with File Manager plugin.

Note: Keep Media CDN disabled.

Step 5. Stop WordPress and enable Media CDN.

For more details, read Shifter MediaCDN article

Step 6. Start WordPress and transfer uploaded media to Shifter CDN.

This backend process may take a while, keep the WordPress dashboard up until all thumbnails appear on the media library page.

Step 7. Rewrite path to the media on your posts with the plugin

Step 8. Regenerate media's thumbnails with regenerating thumbnail plugins after transfer completed.

That's all.

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