A WordPress filter to change a post's GUID from ?p=int to its permalink (use with caution)

set-guid-to-permalink.php

<?php
/**
* Created for Colin Devroe's personal website: cdevroe.com
* It was not his intent that anyone else would use this.
* Please refer to the Migrating WordPress doc which
* emphasizes to **never update guids**
* https://developer.wordpress.org/advanced-administration/upgrade/migrating/#important-guid-note
**/

function namespace_set_guid_to_permalink( $guid, $id ) {
    if ( !is_feed() ) return $guid;
    if ( strpos( $guid, '?p=' ) > 0 ) return get_the_permalink( $id );
}
add_filter( 'get_the_guid', 'namespace_set_guid_to_permalink', 10, 2 );