When you are using the Free Zoiper, you can upgrade it to Zoiper Pro simply by clicking on the "Activate now" button and then providing the credentials with your licenses.
You can activate all your users with just one account containing your licenses, or you can contact our sales team and ask for a reseller account from which you can create sub-accounts for your users. However, in the second case you will be no-longer able to return the given licenses to your pool.
Regarding the third question, we have an F.A.Q. describing how to move licenses to a new computer here: https://www.zoiper.com/en/support/home/article/12/Moving_a_license_to_a_new_device#windows
I can also suggest that you use the contact form on our website(or phone) and ask our sales team directly. They will be able to provide you with more detailed information.