Copy all taxonomy terms into a new taxonomy and re-set them on all posts

snippet.php

<?php
function convert_tax($post_type, $old_tax, $new_tax) {
    $pages = get_posts(array(
        'post_type' => $post_type,
        // Get all post statuses
        'post_status' => array('publish', 'pending', 'draft', 'auto-draft', 'future', 'private', 'inherit', 'trash'),
        'posts_per_page' => -1,
        'tax_query' => array(
            array(
                'taxonomy' => $old_tax,
                'operator' => 'EXISTS'
            )
        )
    ));

    foreach($pages as $post) {
        $terms = get_the_terms( $post->ID, $old_tax );
        $new_terms = array();

        foreach( $terms as $t ) {
            $new_terms[] = $t->slug;
            
            if( get_term_by( 'slug', $t->slug, $new_tax ) == false ) {
                // Clones the terms into the new taxonomy
                // Added slug parameter to account for name and slug being different
                wp_insert_term( $t->name, $new_tax, $args = array('slug' => $t->slug) );
            }
        }
        
        // Re-sets previously set terms
        wp_set_object_terms( $post->ID, $new_terms, $new_tax );
    }
}

convert_tax('company', 'keywords', 'topics');