You do not need to deactivate the old license. You need to contact Autodesk in getting a manual activation code.
To add on to Travis's response, you can utilize the License Transfer Utility to move licenses from one computer to the next, however you need access to both computers. As you do not have access to the old computer, you would need to contact our our License Assistance Team for direct activation support.
To find self-service solutions or reach us via Chat for Activation and Licensing Help:
Visit http://knowledge.autodesk.com/contactus
Select “Activation & Registration” or “License Management”
Select appropriate subtopics from the pull-down menus.
Select “I Still Need Help” if you need personal assistance.
Here to help if additional clarification is needed, otherwise please accept as a solution so that others can benefit from this information.