If the new plan has a different billing frequency from the old plan (for example, monthly to yearly, free to paid, and so on):

The first full period of your new plan begins immediately you will be charged for the first period less any existing prorated account balance.

If the new plan has the same billing frequency, we do something slightly more complex, but with the result of keeping your billing schedule the same:

We immediately create a credit for the remaining portion of your current billing period.

At your next scheduled billing, you will be charged the full charge for the upcoming period, less your account balance (if applicable).

If your account balance amount is larger than the amount due, you will not be charged anything, and your account balance will be carried over to future billing periods, until your balance is spent. If your account balance won't cover the next period, your credit card will be charged for the difference.