The client portal is just a place for you to manage the license keys. It is not connected to your web site in any way and does not do any online checks. So, you can also simply copy the license key into a text file and re-use it as you need. This can be handy if you want to "archive" keys in the client portal - they will be just hidden in the list so you cannot see them. But they are still valid and will work for given version of Kentico.