If you have finished building/migrating your Drupal 8 site and you want your Drupal 7 website to be replaced by the Drupal 8 site, the following guide explains how to do it.
Important: Steps 1-2 are for admins who built their websites in test sites. If you built your D8 site in an official site then skip to step 3.
Create an official Drupal site in web services. The site category should be Official website. The site name does not play a role since it will be replaced by the site name of your Drupal 7 website.
After having created the website, visit
<name_of_D8_official_website>.web.cern.ch/_site/cloneand clone the content of your Drupal 8 test site. So the content should be cloned from your D8 test site to the D8 official site.
After the website is cloned, open a Service Now ticket requesting your Drupal 7 website to be replaced by the Drupal 8 website. In order to boost the process, choose
Drupal Infrastructureas support team. In the ticket you should mention the name of your old Drupal 7 website and also the name of your Drupal 8 website.
In both cases, after the process is done, the URL of your old Drupal 7 website will point to the new Drupal 8 website. Your Drupal 7 will not be lost, it will just have a new URL. For more info regarding it check @kosamara 's comment below.