Licences for Zephyr scale

Hello,

I want to know if the paid licences on Zephyr scale for test management are nominative ? or we can affect, withdraw and reaffect them to users (without of course exceeding the limit of my bought licences) ?

The idea is to buy 20 licences but to have more than 20 who will use them depending on their availability. Today I have 20 QA users and tomorrow I withdraw 5 QA and give their licences to 5 key users. Is this doable ? 

Thank you