Got it worked out, working code as follows:
function action_save_product_data( $post_id ) {
if ( 'product' !== $post->post_type ) {
$terms = wp_get_post_terms( $post_id, 'product_brand', array( 'fields' => 'all' ) );
if ( $terms ) {
$prod_brand = $terms[0]->slug;
wp_set_object_terms( $post_id, $prod_brand, 'product_tag', true );
}
}
}