Moving computer to list patch management

Hi All

We have a number of machines which exceeded total patch management license.

Currently, we are replacing old machines and reinstall agent on new ones. So that we cannot add the new computer to patch mangement license list because we don't know which are current in use and which ones are not.

How can we know which machine was not in use?

How can we move new machine to patch management list?

Thanks and Best Regards