创建php脚本（fpw-swap-thumbnails.php ) 使用以下代码并将其放在站点的根目录中：

<?php
// load WordPress environment
require( \'wp-load.php\' );

$args = array(
    \'posts_per_page\' => -1,
    \'post_type\' => array( \'post\', \'page\' ),
    \'post_status\' => \'publish\'
);

// get all published posts of type specified in $args
$posts = get_posts( $args );

foreach ( $posts as $post ) {
    // get thumbnails ids
    $thmbsArray = array(
        get_post_meta( $post->ID, \'_thumbnail_id\', true ),
        get_post_meta( $post->ID, \'post_secondary-image_thumbnail_id\', true )
    ); 
    // if both thumbnails exist then swap their values
    if ( ( \'\' != $thmbsArray[ 0 ] ) && ( \'\' != $thmbsArray[ 1 ] ) ) {
        update_post_meta( $post->ID, \'_thumbnail_id\', $thmbsArray[ 1 ] );
        update_post_meta( $post->ID, \'post_secondary-image_thumbnail_id\', $thmbsArray[ 0 ] );
    }
}
?>

备份数据库（为了安全起见）。执行脚本。检查结果。删除脚本。