If you have 2 BES servers with the same Configuration Database, you can easy move users from server A to server B. Even when server A is down. If all your users have also BlackBerryOS 4 on the handheld, they get wireless a new service book, and they doesn't notice the move.
You need only a extra server license, no extra/double CAL's.
There are also high availability solution: http://blackberry-links.com/bes-blac...h-availability