What happens will depend on if during your activation you selected to have your brand automatically switched to a premium plan or not.
If you choose to have your card automatically charged at the point where you have used all of your registrations up in the Free subscription, your account will automatically be charged for the plan you chose during your brand activation.
If you chose NOT to have your card automatically charged, fear not, all your data is still safe. You just won’t be able to take any more registrations until you move to a Premium plan.