Drupal 6 to Drupal 8 field conversion helper scripts. Bare bones, rough and simple - yet effective.

D6_D8_sync_db_to_nodes.php

<?php
use Drupal\node\Entity\Node;

$message = "Click button to start Drupal 6 DB to Drupal 8 entityreference field conversion.";

// D6 field machine name
$field = "field_name"; 
// D8 field machine name
$target_field = "field_name";

// Taxonomy entityreference node type machine name
$node_type = "node_type_name";

// D6 Database
$host = "localhost";
$user = "username";
$pass = "thepassword";
$db   = "databasename";

if (isset($_POST['convert'])) {

    $content_field = "content_".$field;
    // when taxonomy ref use instead: $field_value = $field."_value" 
    $field_value = $field."_nid"; 

    // connect to D6 database
    mysql_connect($host, $user, $pass) or die("Could not connect: " . mysql_error());
    mysql_select_db($db);

    // retrieve all D8 nodes
    $nids    = \Drupal::entityQuery('node')->condition('type', $node_type)->execute();
	
    // loop over all D8 nodes
    foreach ($nids as $nid) {
	// load D8 node
        $node = Node::load($nid);
	// retrieve D6 field references for nid
        $result = mysql_query("SELECT `".$field_value."` FROM `".$content_field."` WHERE `nid` = ".$nid);
        $references_array = array();
        while($row = mysql_fetch_array($result)){
             $references_array[] = array(
                            'target_id' => $row[0]
                        );
        }
	// clean up the result
        $references_array = array_values(array_unique($references_array, SORT_REGULAR));
	// set and save the D6 values to the D8 node
        $node->$target_field = $references_array;
        $node->save();
    }
    $message = "Conversion completed.";
}

?>
<br/><br/>
<?php
echo $message;
?>
<br/><br/>
<form  method="post">
<input type="submit" name="convert" value="convert">
</form>

D6_D8_title_based_update.php

Title based sync sql<br/>
<pre>
<?php
use Drupal\node\Entity\Node;

$message = "Ready to convert!";

$field = "field_name"; 
$target_field = 'field_name';

$node_type = 'main_video';

$host = "localhost";
$user = "username";
$pass = 'password';
$db   = "database";

if (isset($_POST['convert'])) {

	$content_field = "content_".$field;
	$field_value = $field."_nid"; // node ref

    mysql_connect($host, $user, $pass) or die("Could not connect: " . mysql_error());
    mysql_select_db($db);

    $nids    = \Drupal::entityQuery('node')->condition('type', $node_type)->execute();
    foreach ($nids as $nid) {
        print($nid.' ');
        $node = Node::load($nid);
		$inner_result = mysql_query("SELECT nid FROM `node` WHERE `title` = '".$node->get("title")->value."'");
		$inner_array = mysql_fetch_array($inner_result);
        $result = mysql_query("SELECT `created` FROM `node` WHERE `nid` = ".$inner_array[0]);
        $row = mysql_fetch_array($result);
		if(isset($row[0]) && $row[0]!=0){
		  $node->set("created",(int)$row[0]);
          $node->save();
		}
    }
	$message = "Conversion completed!";
}

?>
</pre>
<br/><br/>
<?php
if (isset($message)) {
    echo $message;
}
?>
<br/><br/>
<form  method="post">
<input type="submit" name="convert" value="convert">
</form>

D8_tax_lang_post_D6_migrate.php

<?php
use Drupal\node\Entity\Node;

$message = "Ready to convert!";

$node_type = "type_name";
$field_name = "tax_ref_field_name";

if (isset($_POST['convert'])) {
    
    $message = "Conversion has been called.";
    $nids    = \Drupal::entityQuery('node')->condition('type', $node_type)->execute();
    foreach ($nids as $nid) {
        $references_array = array();
        $node = Node::load($nid);
        if ($node->hasTranslation("en") && $node->hasTranslation("nl")) {
            
            // get [en] refs
            $node_en    = $node->getTranslation("en");
            $institutes = $node_en->get($field_name)->referencedEntities();
            
            foreach ($institutes as $inst) {
                if ($inst->hasTranslation("nl")) {
                    $local_tid = $inst->getTranslation("nl")->tid->getValue()[0]["value"];
                    $references_array[] = array(
                        'target_id' => $local_tid
                    );
                }
                if ($inst->hasTranslation("en")) {
                    $local_tid = $inst->getTranslation("en")->tid->getValue() [0]["value"];
                    $references_array[] = array(
                        'target_id' => $local_tid
                    );
                }
            }
            
            // get [nl] refs 
            $node_nl    = $node->getTranslation("nl");
            $institutes = $node_nl->get($field_name)->referencedEntities();
            foreach ($institutes as $inst) {
                if ($inst->hasTranslation("nl")) {
                    $local_tid = $inst->getTranslation("nl")->tid->getValue() [0]["value"];
                    $references_array[] = array(
                        'target_id' => $local_tid
                    );
                }
                if ($inst->hasTranslation("en")) {
                    $local_tid = $inst->getTranslation("en")->tid->getValue() [0]["value"];
                    $references_array[] = array(
                        'target_id' => $local_tid
                    );
                }
            }
            
            $references_array = array_values(array_unique($references_array, SORT_REGULAR));
            
            // save all to both
            $node_nl->$field_name = $references_array;
            $node_nl->save();
            $node_en->$field_name = $references_array;
            $node_en->save();
        }
    }
}
?>
<br/><br/>
<?php
echo $message;
?>
<br/><br/>
<form  method="post">
<input type="submit" name="convert" value="convert">
</form>