Once a cover has been assigned to an album, Centova Cast will not attempt to replace it even if you change your album data source priorities. This was a deliberate design decision; most folks don't want CC spending half an hour re-downloading the album data for their entire library every time they change their album data source settings.
You would need to remove the cover from the existing album (right-click on it in the media library to do so) if you want to replace it.