You can use the BlackBerry Enterprise Transporter when your organization upgrades the BlackBerry® Enterprise Server and you want to create a new BlackBerry Domain, or if you need to move user accounts between BlackBerry Domain instances. For example, if you want to upgrade your organization’s BlackBerry Enterprise Server from version 4.0 SP7 to version 5.0, you can create a separate BlackBerry Domain version 5.0 and use the BlackBerry Enterprise Transporter to move your organization’s user accounts to the new BlackBerry Configuration Database. You can also use the BlackBerry Enterprise Transporter to move user accounts from a production BlackBerry Domain to a test BlackBerry Domain and back.
You need to do the enterprise activation again which means this process is transparent to the user community and they will not notice that they have moved to a different server. I have also tested with the users who had more approx. 4000 contacts and it worked smoothly. Before you migrate the users you must create a manifest file and preview the migration process.
Source URL- http://docs.blackberry.com/en/admin/deliverables/7534/BB_ET_overview_554166_11.jsp