Change existing or add custom title or meta template variables

yoast_seo_title_change-variable.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/
/* Remove Yoast SEO Change existing title or meta template variable
 * Credit: Moshe Harush
 * https://stackoverflow.com/questions/36281915/yoast-seo-how-to-create-custom-variables
 * Last Tested: Unknown
 */
 
 // define the wpseo_replacements callback
function filter_wpseo_replacements( $replacements ) {
    if( isset( $replacements['%%page%%'] ) ){
        $replacements['%%page%%'] = 'Page x of y';
    }
    return $replacements;
};
// Add filter
add_filter( 'wpseo_replacements', 'filter_wpseo_replacements', 10, 1 );

yoast_seo_title_custom-variable.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/
/* Remove Yoast SEO Add custom title or meta template variables
 * Credit: Moshe Harush
 * https://stackoverflow.com/questions/36281915/yoast-seo-how-to-create-custom-variables
 * Last Tested: Nov 29 2018 using Yoast SEO 9.2.1 on WordPress 4.9.8
 *******
 * NOTE: The snippet preview in the backend will show the custom variable '%%myname%%'.
 * However, the source code of your site will show the output of the variable 'My name is Moses'.
 */

// define the custom replacement callback
function get_myname() {
    return 'My name is Moses';
}

// define the action for register yoast_variable replacments
function register_custom_yoast_variables() {
    wpseo_register_var_replacement( '%%myname%%', 'get_myname', 'advanced', 'some help text' );
}

// Add action
add_action('wpseo_register_extra_replacements', 'register_custom_yoast_variables');

if someone needs to replace the custom variables also in the backend snippet preview, this is an example
https://gist.github.com/erikyo/53ec6a1b2264135dae95e866194a0a8a

How would I duplicate an existing template variable and modify it?

Or, in other words, how can I access $replacements['%%category_description%%'] in my replacement callback function?