Copy/Paste from the very user friendly User Manual:
"...Transferring Licenses
If you would like to transfer your license from one machine to another, simply unactivate the currently licensed machine. This process is similar to the licensing procedure, but in reverse. If the licensed Ignition server has internet access, the unactivation will occur immediately, and the license will again be available for activation. If you do not have internet access, an unactivation request file will be generated, and the license will not be allowed to activate until the file is loaded into the Inductive Automation website, or emailed to support..."