add_filter( 'wp_generate_attachment_metadata', 'replace_uploaded_image' );

function replace_uploaded_image($image_data)
{
    // abortar se não houver versão "large" da imagem
    if ( !isset($image_data['sizes']['large']) )
        return $image_data;

    // paths do upload e da imagem grande
    $upload_dir              = wp_upload_dir();
    $uploaded_image_location = $upload_dir['basedir'] . '/' . $image_data['file'];
    $large_image_location    = $upload_dir['path'] . '/' . $image_data['sizes']['large']['file'];

    // deletar original
    unlink($uploaded_image_location);

    // renomear a imagem "large"
    rename($large_image_location, $uploaded_image_location);

    // atualizar o metadata da imagem e retornar
    $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_image_size( 'new-large', 1600, 1200 );