Yes, the licenses you purchased are transferable. This means that you are able to remove an existing member 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.
After a learner's removal from a Program, all data tied to that Learner will be deleted from admin reports. Removed learner accounts will transition to consumer accounts that are not sponsored by a company's Coursera instance. All learner achievements including Certificates and course progress will be preserved and available to the learner via their consumer account. Should the learner wish to enroll in additional courses, they will have the option to do so. They will also be requested to provide payment to attain Certificates post course completions where a Certificate is offered.