Update the user_nicename field in the database after changing a username.

update-user-nicename-after-username-change.php

<?php
/**
 * Update the user_nicename field in the database after changing a username.
 *
 * Use this snippet after updating a user's username (user_login) using a plugin like
 * "Easy Username Updater" or similar tools. This ensures that the profile URL updates
 * correctly by setting the user_nicename field to match the updated username.
 *
 * Instructions:
 * 1. Add this snippet to your theme's functions.php file or use a code snippets plugin.
 * 2. Replace 'USER_ID_HERE' with the actual user ID of the member whose URL you want to update.
 * 3. Save the changes and visit the site to execute the code.
 * 4. IMPORTANT: Remember to remove or disable the snippet after execution to prevent unintended updates.
 *
 * Read more about customizations:
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

add_action('init', function() {
    $user_id = USER_ID_HERE; // Replace with the user's ID.
    $user = get_userdata($user_id);

    if ($user) {
        // Sanitize the username to create a URL-friendly user_nicename.
        $sanitized_nicename = sanitize_title($user->user_login);

        // Update the user_nicename in the database.
        wp_update_user([
            'ID' => $user_id,
            'user_nicename' => $sanitized_nicename,
        ]);

        // Flush rewrite rules to update the URL structure.
        flush_rewrite_rules();
    }
});