[Really Simple CSV Importer] add-on: Update row based on a custom field ID/key match

importer-class-filter-example.php

<?php
/*
Plugin Name: Update row based on a custom field ID/key match
Plugin URI: https://wordpress.org/support/topic/update-row-based-on-a-custom-field-idkey-match
*/

add_filter('really_simple_csv_importer_class', function() {
    return "ImporterCustomize";
});

class ImporterCustomize
{
    public function save_post($post,$meta,$terms,$thumbnail,$is_update)
    {
        /* Start customization */
        
        // Get the identifier meta value
        $sku_value = $meta['SKU'];
        
        // Get a matched post by meta value
        $args = array(
            'post_type' => 'post',
            'post_status' => 'any',
            'meta_query' => array(
                array(
                    'key'              => 'SKU',
                    'value'            => $sku_value,
                    'compare'          => '=',
                    'cache_results'    => false,
                    'suppress_filters' => true
                )
            )
        );
        $query = new WP_Query( $args );
        if ( $query->have_posts() ) {
            $original_post = $query->next_post();
            
            // Add ID value for replacement post data
            $post['ID'] = $original_post->ID;
            $is_update = true;
        }
        
        /* End customization */
        
        /* ///////////////// */
        
        /* Copied from RS_CSV_Importer#save_post */
        
        // Separate the post tags from $post array
        if (isset($post['post_tags']) && !empty($post['post_tags'])) {
            $post_tags = $post['post_tags'];
            unset($post['post_tags']);
        }
        
        // Add or update the post
        if ($is_update) {
            $h = RSCSV_Import_Post_Helper::getByID($post['ID']);
            $h->update($post);
        } else {
            $h = RSCSV_Import_Post_Helper::add($post);
        }
        
        // Set post tags
        if (isset($post_tags)) {
            $h->setPostTags($post_tags);
        }
        
        // Set meta data
        $h->setMeta($meta);
        
        // Set terms
        foreach ($terms as $key => $value) {
            $h->setObjectTerms($key, $value);
        }
        
        // Add thumbnail
        if ($thumbnail) {
            $h->addThumbnail($thumbnail);
        }
        
        return $h;
    }
}