When you upgrade your account on a monthly plan, you'll be charged and your billing date will be moved to the day of the change.
This means you may be charged twice within the same calendar month, to cover over a month of subscription.
For example, let's say your original billing date is on the 1st of the month and you upgrade your Simple monthly plan to Modern on the 24th. The system will apply the prorated amount left from the original charge, then immediately charge a full month's subscription.
The total monthly amount will cover from the 1st of the month, until the 24th of the FOLLOWING month. This means that two payments have been taken during the month that cover nearly TWO months of your subscription.