No need to delete the user. On BES, right click on the users name, go to Properties. Once the window open, choose 'password activation' (or something like that) enter a password (123 is probably best).
Now, on the users new device, they go to 'enterprise activation', enter their email address (for the company / bes domain), and the password of '123', and then 'activate'. Within minutes everything should sync wirelessly...
There is no 'big' deal if you remove the account, but you have to ask yourself, "Why do that?"
If you do remove the account, it will ask do you want to remove all history or BlackBerry details from the users account. Choosing yes/no is a personal choice...
Now, dismissing all the above - if for example the person wants to keep all that 'extra' data from their present device (ringtones, certain settings, etc), then I suggest:
1. ...doing a backup of the device on their pc.
2. ...switch sim card to new device.
3. ...if the pc is connected to the domain, run desktop manager.
4. ...connect the new device.
5. ...BES should see it, and add him, and it will ask for a new encryption key to be generated (you have to move the mouse to generate this).
6. ...once done, i.e. connected to BES, do a restore from desktop manager.
Clear as mud?
