You have to delete the monitors that you no longer require. Players of deleted monitors will be de-registered and the monitors will constantly display a relevant message, waiting for registration.
If you downgrade without deleting the extra monitors you no longer require, then Xandadeck will automatically deactivate the last monitors you added in order to match your subscription limit. You can then go back and delete the monitors you actually no longer require, and inactive monitors will automatically come back online.
Charges are fully automated, so you will be billed (or credited) the difference of your plan, so do not worry about losing service credit.