If a member of my team stops working for me, may I assign a different staff member to replace her?

Yes, you may. When you purchase a team subscription, you sign up for a certain number of seats. For any reason, you may remove someone from your team and replace them with someone else. You do this by logging in to your account and navigating to Teams on your My Account dashboard. See the screenshot below of a team owner/manager dashboard with Teams highlighted.

After clicking on the Teams option, you will see a new page with a choice for Members. Navigate to Members where you have the option of removing a team member. After removing a member, navigate to the Add Member page. Here you can add a team member by Registration Link or an Email Invitation.