We bill at the END of each period not at the beginning. This means if you use an extra register or upgrade to higher plan you cannot go down until the next period and you will be charged for that usage during the current period.
So be sure you want to add that extra register before you do it.
To downgrade your plan or remove a register for the next period you need to first make sure the register is in the "Available" state. See this posting on how to do that.
Next go to Settings -> Account Settings and click on CHANGE PLAN
Then towards the bottom right click CHANGE to view your plan for the next period.
Now you can set your additional registers and downgrade your plan for the next billing period here. Don't forget to hit UPDATE.
