To re-register TrueConf Server License is quiet simple. After TrueConf support Team confirm that they had update the license. You can re-use the same license as before to re-register TrueConf Server.
Go to Dashboard > Summary. At right corner , you will register button . Click it.
Enter same license that you use during first time activation. (Re-use same license code).
and then click "Register" .