Yes, you can change and re-allocate those who occupy paid contributor seats (licenses) in your workspace at any time.
Workspace admins can rotate and update user access roles on their account through the user settings. With this, admins can update a previous contributor's workspace role to a "Viewer" role (or remove from completely from workspace) once a new contributor is invited to the workspace.
For example, if Person A has the workspace role of a "User" but no longer requires this seat, you can change their role to a "Viewer". This will free up a seat where you can invite Person B to become a "User" in the workspace and take this seat.
When amending the allocation of the contributors, if you are unsure of the amount of seats available you can also review current usage in the billing settings.