It is not possible to transfer a BlackBerry Messenger PIN from one phone to another. This is because the PIN refers to the individual handset, not the user or account. However, if you switch from one phone to another, you can use software to automatically make sure your friends are still able to reach you through BBM.
Strictly speaking, the PIN in question is a BlackBerry pin rather than a BBM pin. It is a specific code that is built into the handset itself and cannot be transferred to another handset. The reason it is known as a BlackBerry Messenger PIN is that users can send messages over BlackBerry's network direct to a handset using its PIN. This removes the need to give somebody your phone number or e-mail address before they can contact you.
If you get a new BlackBerry handset, you can use the BlackBerry Desktop Software to transfer your settings. To do so, connect your old handset to your computer via USB cable, open the BlackBerry Desktop Software, click on "Device" then "Switch device" and follow the onscreen instructions. Your computer will back up your handset data then ask you to disconnect the handset and connect your new handset, transferring the data.
When you transfer your settings in this way, the BBM system automatically updates your listed details for all your contacts. They then will be able to select your name in their contacts list and will automatically be connected to your new PIN and thus get the message through to your new handset. Remember that you'll still need to give the new handset PIN to any new contacts you make. To find the new handset pin select "Options" then "Status" and look for an eight-character PIN.
BlackBerry handsets sold from 2013 and running the new BlackBerry 10 operating system do not need to use a PIN. Instead users have a BlackBerry ID, which is a username and password that identifies the user rather than the handset. The user can change to a new handset but keep the same ID and continue receiving messages uninterrupted.
Source: GlobalPost (saved me typing out essentially the same thing)