Automatically replace original uploaded image with large image size

Add this to the functions.php file in the theme folder. It replaces the original image with the large image set in settings. You might want to setup a new image format and use that as the new original size though.

function replace_uploaded_image($image_data) {
      // if there is no large image : return
  if (!isset($image_data['sizes']['large'])) return $image_data;

  // paths to the uploaded image and the large image
  $upload_dir = wp_upload_dir();
  $uploaded_image_location = $upload_dir['basedir'] . "https://wordpress.stackexchange.com/" .$image_data['file'];
  // $large_image_location = $upload_dir['path'] . "https://wordpress.stackexchange.com/".$image_data['sizes']['large']['file']; // ** This only works for new image uploads - fixed for older images below.
  $current_subdir = substr($image_data['file'],0,strrpos($image_data['file'],"https://wordpress.stackexchange.com/"));
  $large_image_location = $upload_dir['basedir'] . "https://wordpress.stackexchange.com/".$current_subdir."https://wordpress.stackexchange.com/".$image_data['sizes']['large']['file'];

  // delete the uploaded image
  unlink($uploaded_image_location);

  // rename the large image
  rename($large_image_location,$uploaded_image_location);

  // update image metadata and return them
  $image_data['width'] = $image_data['sizes']['large']['width'];
  $image_data['height'] = $image_data['sizes']['large']['height'];
  unset($image_data['sizes']['large']);

  return $image_data;
}

add_filter('wp_generate_attachment_metadata','replace_uploaded_image');