You will need to drop your student from their current class and then have the join the new one using the class key for that class. This won't affect your overall amount of seats used. Work does not go with the student. If you or the student want a copy of that work, then it will need to be printed to paper or PDF prior to dropping them.
If your class is set up as Student Pay class, your student will be prompted for payment again when joining the new class. You or your student can request a refund for the original seat, which you can do using our chat system. We'll need the email address for the student, and the refund will appear in their account in 5-10 days.