When the zDE is upgraded and new installation is taking place, following are the possibilities:

• If zDE is running, it is stopped, upgraded and restarted.
• If your sites are running, each site is shut down before migration and then restarted.
• If your site is not running, it is not started, but the site is upgraded.
  This makes sure that you are not thrown off the zDE domain when up gradation and subsequent installation takes place.