WordPress Move Meta data from OLD custom field Key to New key

functions.php

<?php 
# Inlcude in functions file for one time
# Remove after calling this function on some page one time

function warmarks_update_new_meta_from_old(){
  
  # Provide your custom values here:
  $post_type = 'pdf'; //change it with your post type
  $old_metaKey = 'ecpt_pdflink';
  $new_metaKey = '_jb_pdf';

  
    
$args = array( 'posts_per_page' => -1, 'post_type' => $post_type );
$posts = get_posts($args);

foreach ($posts as $post) {
  # code...
  $old_link = get_post_meta( $post->ID, $old_metaKey, true );
  $new_link = get_post_meta( $post->ID, $new_metaKey, true );

if($old_link != '' && $new_link == ''){
  update_post_meta($post->ID, $new_metaKey, $old_link);
}
  
}

}