Copy content stored in meta to post content

Here’s an example of how to get the meta and set it as the post content. You would need to add all the meta keys to the $meta_fields array.

Obviously test this locally before running in production because it will overwrite whatever is currently in the post content.

function migrate_post_data() {   
    // Add all the meta keys you want to migrate to this array.
    $meta_fields = array(
        'flexible_content_1_content',
        'flexible_content_2_content'
    );

    // Get all the posts.
    $args = array(
        'post_type'      => 'post', // You may need to change this.
        'posts_per_page' => 400,
    );
    $posts = get_posts( $args );

    // Loop over each post.
    foreach ( $posts as $post )  {
        $meta_field_content = array();

        // Loop over each meta field and get their content, adding it to an array.
        foreach( $meta_fields as $meta_field ) {
            $content = get_post_meta( $post->ID, $meta_field, true );

            if ( ! empty( trim( $content ) ) ) {
                $meta_field_content[] = $content;
            }
        }

        if ( empty( $meta_field_content ) ) {
            continue;
        }

        // Set the the post content as whatever the meta fields were.
        $post_args = array(
            'ID' => $post->ID,
            'post_content' => implode( ' ', $meta_field_content ),
        );

        wp_update_post( $post_args );
    }
}