Hi EXV, sorry to hear you're running into issues with managing your seat settings!
Our support team is best equipped to review your account and confirm what’s possible in your specific case. (For privacy and security reasons, we're not able to handle these requests publicly in the Forum).
I see in the backend that you have already reached out to the support team, I’ve escalated it internally.
Also, I recommend you to check out Manage billing on the Professional plan for additional information on how seats will be charged with the updated system.
Hope this helps, and thanks for your patience in the meantime! 
Update: The support team is waiting for your response, please have a look to your inbox when you have a moment. Thanks!