Migrate terms from old taxonomy to new taxonomy and assign respective posts.

class-migrate-category.php

<?php

/**
 * Go to /wp-admin/edit-tags.php?taxonomy={initial_tax}&post_type={post_type} and paginate in categories to execute script.
 * You can delete this file once it's done
 * Class Migrate_Category
 */
class Migrate_Category
{
    const TAXONOMY_COLUMN = 'data'; // Column name
    /**
     * @var string
     */
    protected $initial_tax;
    /**
     * @var string
     */
    protected $target_tax;
    /**
     * @var string
     */
    protected $post_type;
    /**
     * @var WP_Term
     */
    private $cloned_term;

    /**
     * Migrate_Category constructor.
     *
     * @param $initial_tax
     * @param $target_tax
     * @param $post_type
     */
    public function __construct($initial_tax, $target_tax, $post_type)
    {
        $this->initial_tax = $initial_tax;
        $this->target_tax  = $target_tax;
        $this->post_type   = $post_type;

        add_action('manage_edit-category_columns', [$this, 'add_column'], 1);
        add_filter("manage_category_custom_column", [$this, 'column_content'], 10, 3);
    }

    /**
     * Column is used to get access to term_id and output data
     *
     * @param $columns
     *
     * @return mixed
     */
    public function add_column($columns)
    {
        $columns[self::TAXONOMY_COLUMN] = self::TAXONOMY_COLUMN;

        return $columns;
    }

    /**
     * @param $value
     * @param $column_name
     * @param $id
     */
    public function column_content($value, $column_name, $id)
    {
        $reset = false;

        if ($column_name == self::TAXONOMY_COLUMN) {
            // Delete new terms in dev
            if ($reset) {
                $terms_delete = get_terms([
                    'taxonomy'   => $this->target_tax,
                    'hide_empty' => false,
                ]);
                if ($terms_delete) {
                    foreach ($terms_delete as $term_n) {
                        if (term_exists($term_n->term_id, $this->target_tax)) {
                            wp_delete_term($term_n->term_id, $this->target_tax);
                        }
                    }
                }
                // Cache entries
                delete_option($this->target_tax . '_children');

                return;
            }

            $term = WP_Term::get_instance($id, $this->initial_tax);

            $value .= $id;
            if (! $term || is_wp_error($term)) {
                return;
            }

            if (! term_exists($term->slug, $this->target_tax) && (get_term_by(
                'slug',
                $term->term_id,
                $this->target_tax
            ) == false)) {
                if ($term->parent === 0) {
                    // add top level first to make sure they are created one we assign the children parent logic
                    $this->clone_term($term, $this->target_tax, true);
                } else {
                    $this->clone_term($term, $this->target_tax);
                }

                if ($this->cloned_term) {
                    $this->assign_posts($term, $this->cloned_term);

                    ?>
                    ✔ <a target="_blank"
                         href="<?php echo admin_url("term.php?taxonomy=" . $this->cloned_term->taxonomy . "&tag_ID={$this->cloned_term->term_id}&post_type=" . $this->post_type . ""); ?>"><?php echo $this->cloned_term->term_id ?></a> ==
                <?php }
            }
        }

        echo $value;
    }

    /**
     * @param WP_Term $term
     * @param string $target_taxonomy
     * @param bool $is_top_level
     */
    public function clone_term(WP_Term $term, $target_taxonomy, $is_top_level = false)
    {
        if (! $term instanceof WP_Term || $term->term_id == 1) {
            return;
        }

        $args = [
            'description' => $term->description,
            'slug'        => $term->slug
        ];

        if (! $is_top_level) {
            // Find parent object
            $term_parent = get_term_by('id', $term->parent, $term->taxonomy);

            if ($term_parent) {
                // We find his cloned version for the new taxonomy
                $cloned_term_parent = get_term_by('slug', $term_parent->slug, $target_taxonomy);

                if ($cloned_term_parent) {
                    // Assign to term args
                    $args = array_merge($args, [
                        'parent' => $cloned_term_parent->term_id
                    ]);
                }
            }
        }

        // Create new term
        wp_insert_term($term->name, $target_taxonomy, $args);

        // New term is available
        $cloned_term = get_term_by('slug', $term->slug, $target_taxonomy);

        if ($cloned_term) {
            // Copy extra data once the term is created
            $this->cloned_term = $cloned_term;

            $acf_field_icon   = 'svg_icon';
            $term_icon        = get_field($acf_field_icon, $term->taxonomy . '_' . $term->term_id);
            $cloned_term_icon = get_field(
                $acf_field_icon,
                $this->cloned_term->taxonomy . '_' . $this->cloned_term->term_id
            );

            // Acf field exists in old and is null in new term
            if ($term_icon && ! $cloned_term_icon) {
                update_field(
                    $acf_field_icon,
                    $term_icon,
                    $this->cloned_term->taxonomy . '_' . $this->cloned_term->term_id
                );
            }
        }
    }

    /**
     * @param WP_Term $term
     * @param WP_Term $cloned_term
     */
    public function assign_posts(WP_Term $term, WP_Term $cloned_term)
    {
        // All post using initial term/taxonomy
        $post_args = [
            'posts_per_page' => -1,
            'post_type'      => $this->post_type,
            'tax_query'      => [
                [
                    'taxonomy'         => $term->taxonomy,
                    'field'            => 'term_id',
                    'terms'            => $term->term_id,
                    'include_children' => false
                ]
            ]
        ];

        $posts = get_posts($post_args);
        if ($posts) {
            foreach ($posts as $post) {
                // Verify that the term exists and has not been attached to current post
                if ((bool)term_exists(
                    $cloned_term->term_id,
                    $cloned_term->taxonomy,
                    ($cloned_term->parent !== 0 ? $cloned_term->taxonomy : null)
                ) && ! has_term(
                    $cloned_term->slug,
                    $cloned_term->taxonomy,
                    $post->ID
                )) {
                    // Set cloned term to post
                    wp_set_object_terms($post->ID, $cloned_term->term_id, $cloned_term->taxonomy, true);
                }
            }
        }
    }
}

$migrate_category = new Migrate_Category('category', 'faq_category', 'hrf_faq');