it should be pretty straightforward.
Install your new BES server making sure to use the same database. (That way they will be able to see each other)
Move a couple of test devices to the new sever from the old and verify.
Move ther other devices to the new server.
*The database is the key in these installs
*Be sure that you have the database in a central location so that when you remove the old server you dont remove the database database with it