Is there a fee or charge if I downgraded from Professional Plan to FREE Plan?

Is there a fee or charge if I downgraded from Professional Plan to FREE Plan?

When I upgraded to Professional Plan, I paid the necessary amount. But then, my other user/agent have been removed. If I'm going to downgrade again to FREE Plan, will there be a charge? After some time, If I decided to go back to my Professional Plan will I pay again the amount for upgrade? Will I pay the same amount for upgrade if I want to add user or agent?