Change WordPress comments url / word

In your functions.php add ( there are other rules relating to comment pages you can see them all with Rewrite Rules Inspector plugin, this one just covers the case you mention )

add_rewrite_rule (
  '(.?.+?)/customname-([0-9]{1,})/?$', 
  'index.php?pagename=$matches[1]&cpage=$matches[2]', 
  'top'
);

you’ll also need to find in your theme ( possibly in your function.php ) where the comments are generated and change where the id is added to the comment html from something like ( if you can specify what theme your using can be more specific )

<li <?php comment_class('clearfix'); ?> id="comment-<?php comment_ID(); ?>">

to

<li <?php comment_class('clearfix'); ?> id="customname-<?php comment_ID(); ?>">