migration_helpers/migration_helpers.module

<?php

use Psr\Log\LogLevel;

/**
 * Entity query helper.
 */
function migration_helpers_entity_query(
  $entity_type = 'node',
  $bundle = NULL,
  $field = NULL,
) {

  /** @var \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager */
  $entity_type_manager = \Drupal::service('entity_type.manager');

  $entity_storage = $entity_type_manager->getStorage($entity_type);

  $entity_query = $entity_storage->getQuery();

  if ($bundle) {
    $bundle_key = $entity_storage->getEntityType()->getKey('bundle');
    $entity_query->condition($bundle_key, $bundle);
  }

  if ($field) {
    // Check that field BOTH is present on entity bundle AND has a value saved.
    $entity_query->exists($field);
  }

  $entity_query->accessCheck(FALSE);

  $entity_ids = $entity_query->execute();

  return $entity_type_manager->getStorage($entity_type)->loadMultiple($entity_ids);

}

/**
 * Move (and optionally filter/transform) terms between term reference fields.
 */
function migration_helpers_move_terms(
  $destination_vocabulary,
  $source_field,
  $destination_field,
  $bundle = NULL,
  $entity_type = 'node',
  $mapping = [],
) {

  /** @var \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager */
  $entity_type_manager = \Drupal::service('entity_type.manager');

  // Get any existing terms in destination vocabulary so we can efficiently
  // prevent duplication.
  $terms = migration_helpers_entity_query(
    entity_type: 'taxonomy_term',
    bundle: $destination_vocabulary,
  );

  $term_data = [];
  foreach ($terms as $term) {
    $term_data[$term->id()] = $term->label();
  }

  $nodes = migration_helpers_entity_query($entity_type, $bundle, $source_field);
  foreach ($nodes as $node) {
    $tids = [];
    /** @var \Drupal\node\NodeInterface $node */
    $orig_values = $node->get($source_field);
    foreach ($orig_values as $orig_value) {
      $orig_tid = $orig_value->target_id;
      $orig_term = $entity_type_manager->getStorage('taxonomy_term')->load($orig_tid);
      $orig = $orig_term->name->value;
      $term_name = $mapping[$orig] ?? $orig;
      // If our map intentionally blanked out the term name, do not create
      // a term in the new vocabulary and assign it to the destination field.
      // Note, the term must be blanked out ('') or assigned to FALSE in the
      // mapping, setting a term to NULL is the same as not mapping it, and it
      // would be created in the new vocab etc.
      if (!$term_name) {
        continue;
      }
      $tid = array_search($term_name, $term_data);
      if (empty($tid)) {
        $new_term = $entity_type_manager->getStorage('taxonomy_term')->create([
          'name' => $term_name,
          'vid' => $destination_vocabulary,
        ]);
        $new_term->save();
        $tid = $new_term->id();
        \Drupal::logger('mass')->log(LogLevel::INFO, "Term $term_name created with TID $tid in vocabulary $destination_vocabulary.");

        // Because we did the query only once, before we entered this loop, we
        // need to add our newly created terms to the term data array we use to
        // check for existing terms… or we'll keep creating duplicates.
        $term_data[$new_term->id()] = $new_term->label();
      }
      $tids[] = $tid;
    }
    // Now we have collected all the term IDs that have been created.
    // We have to go through a surprising number of hoops to make our
    $already_there_tids = [];
    $already_there_values = $node->get($destination_field);
    foreach ($already_there_values as $already_there_value) {
      $already_there_tids[] = $already_there_value->target_id;
    }
    $node->set($destination_field, array_merge($tids, $already_there_tids));
    $node->save();
  }
}

// The below almost serve more as examples that can be followed

function migration_helpers_text_list_to_boolean($old_field_name, $new_field_name, $bundle, $entity_type = 'node', $value_map = NULL) {

  $value_map = $value_map ?? [
    'Yes' => TRUE,
    'No' => FALSE,
  ];

  $nodes = migration_helpers_entity_query($entity_type, $bundle, $old_field_name);
  foreach ($nodes as $node) {
    /** @var \Drupal\node\NodeInterface $node */
    $orig_value = trim($node->$old_field_name->value);
    $new_value = $value_map[$orig_value] ?? (boolean) $orig_value;
    $node->set($new_field_name, $new_value);
    $node->save();
  }
}