Get posts by custom taxonomy and resave them programmatically

<?php

// Get all posts by custom taxonomy

$a = get_posts(array(
    'showposts' => -1, // you can try and test run this by setting a pagination value of higher than 1. 
    'post_type' => 'post',
    'tax_query' => array(
        array(
        'taxonomy' => 'taxonomy-name',
        'field' => 'name',
        'terms' => array('taxonomy-slug'))
    ),
    'orderby' => 'ID',
    'order' => 'ASC')
);

$posts = array();

// Generate array of post IDs
 
foreach ( $a as $post ) {
   $posts[] += $post->ID;
}

// Feed post array into wp_update_post

foreach ( $posts as $post_id ) {

$arg = array(
    'ID'            => $post_id,
);
wp_update_post( $arg );

}