How is prorate calculated?
When you upgrade a plan at any given time, the system will calculate the prorated price to make sure you're not overpaying.
First, the system will calculate the per-day amount owed by the customer for the lower-tier (current) plan. This per-day amount is then multiplied by the number of days remaining in that billing cycle, and we get the amount that will be refunded to your account in the form of a credit note.
Next, the system will calculate the per day charge of the higher-tier plan and multiply by the number of days remaining in that billing cycle. This is the additional amount that you will be paying for the new plan.
Let's say you are on the Basic plan, which is $10.00 / month. You started the subscription on September 1st but decided to upgrade to Pro plan, which is $99.00 / month on September 15th, so the numbers will look like this:
Total days in the billing cycle - 30 days
Basic plan - ($10.00/30) = $0.33 / day
Pro plan - ($99.00/30) = $3.30 / day
Days remaining on the billing cycle - 15 days
Refund (Basic plan) - $0.33 * 15 = $4.95
Upgrade charge (Pro plan) - $3.30 * 15 = $49.5
With this example, you'll get a $4.95 refund in the form of a credit note, and you'll be charged $49.5 upon upgrade.