Yes, the licenses you purchased are transferable. This means that you are able to remove an existing member (once you remove them their data will be deleted) and use up that freed up license to invite a new learner.
Usually administrators remove those who are not active to give other team members a chance to learn.
You can change the current set of members throughout the year as many times as you want. Also, keep in mind that you can send unlimited number of invitations to fill the seats you've purchased.