You can pull the image, tag it and push it to the new registry.
Example:
docker pull old-registry/app:some_tag
docker tag old-registry/app:some_tag new-registry/app:some_tag
docker push new-registry/app:some_tag
You can pull the image, tag it and push it to the new registry.
Example:
docker pull old-registry/app:some_tag
docker tag old-registry/app:some_tag new-registry/app:some_tag
docker push new-registry/app:some_tag