Which existing image and thumbnail URL in the database?
Such thing doesn't exists as the URL is dynamic, taken from the storage URL. If you move to another storage and you keep the same domain then all the URLs will keep working.
like Rodolfo said, if you enable a external storage it will automaticly start reading thumbnails from external storage. Also existing images does get put on external storage as well. you just need to upload existing ones over to external storage!
Which existing image and thumbnail URL in the database?
Such thing doesn't exists as the URL is dynamic, taken from the storage URL. If you move to another storage and you keep the same domain then all the URLs will keep working.