All new users are added immediately to the free plan after they sign up but at any time you may upgrade your account to get additional features and higher capacity. If you’re upgrading from the free plan to a paid plan you’ll be billed the full amount of the new plan at the time of the upgrade. If you’re upgrading from one paid plan to a higher one, we’ll bill you for the prorated difference between your current plan and the new one. This proration will be charged at the time of the upgrade.
Here's a real-world example. Let's say you purchase the $19/month plan on July 1st and then decide to upgrade to the $49/month plan halfway through your current billing cycle (July 15). In this case, the you have already paid $19, and used half of the current billing cycle. When you switch the new plan, with a cost of $49, the unused portion of the previous plan results in a credit of $9.50. The cost of the new plan for the rest of the billing cycle (i.e., the rest of July) will be $24.50. Therefore, the total prorated cost of switching from the lower plan to the higher plan is $15.00. This amount will be billed immediately at the time of the upgrade.