After enable W3 Offline Media you only can sync the new media, the old ones in your library must use a paid plugin.
With my hack solution, it is simple, just enable my plugin and the previous attachment you have the URL modified on fly to bucket location.
- Enable Wp Offline Media
- Upload Hack Plugin
- Configure the Hack Plugin (direct on code)
- Enable plugin
Remembter to sync your upload folder on S3 before enable the plugin