@mmmmm Before trying to preempt a job scheduler runs a simulation where it checks whether or not preempting a job will release enough resources to run the higher priority job.
In the case you mentioned, it should work just fine as long as your server_dyn_res script reports the license count as the sum of “free licenses + licenses consumed by jobs that PBS ran” (as mentioned in your previous post by Bhroam - [Preempt queue] Jobs in queue1 should stop to let jobs in queue2 start )