Yes, you can convert your annual plan to an month-to-month plan at any time in the Account section of the site. To do so, go to the Billing section, click the Billing Information tab and select the Change Plan button to complete the process.
The monthly billing anniversary begins immediately. If your account balance is larger than the amount due, you will not be charged anything. Your account balance is carried over to future billing periods, until it’s spent down. If your account balance doesn't cover the monthly charge, you will be charged the difference between your balance and the charge.