To transfer the license, visit the Boomerang Web Management Dashboard and then delete the old device by clicking on the hammer and screwdriver near the old device name. This will free up that license. Then go to the licensing section which is under Account/Licensing in the web management dashboard and here use the drop down to apply the license to the new device.
Note: Sometimes a device can’t be deleted immediately; if so, try again and on the 2nd or 3rd attempt, it will get deleted. If you have any issue, just let me know and I will go into our backend and do it for you.