Deactivate
When you go to the Help menu and pick "Deactivate", kJams will put one or more "serial number" files on your desktop. There will be one file for the kJams application (either Lite, Pro, or 2), and one each for the Burner and Producer if you had purchased them as well.
Copy these files to the new computer. On the new computer, run kJams, then go to the Help menu and pick "Reveal Preferences", then Quit kJams. Then, drag the serial number file(s) into the Preferences folder that was revealed, then you can close it.
Now run kJams again, and it will automatically activate using your serial number file(s).