We're using cost proration, it basically means that you're paying for the time you spent on each plan during the billing cycle. Note that all the invoices are always billed at the start of the billing cycles so when you upgrade you're never billed right away.
Below are a few examples to demonstrate how exactly it works.
You purchased plan $99 on August 15th, you were charged $99 for the August 15 - September 15 billing cycle. Plan $99 has $10k ad spend limit. In the middle of the billing period you decided to upgrade to the plan $199. As mentioned above you will not be charged anything right away, instead the next invoice about will be increased by the difference between what you've already paid and what you owe for the bigger plan. The math here is pretty simple: you used plan $99 for half a month and for another half you were on the $199 plan, so the total cost for the August 15 - September 15 period should be the sum of the halves of these plans.
99/2 + 199/2 = $149 - this is how much this particular billing period will cost.
But you have already paid $99 for this billing cycle, which means the next invoice amount will be increased by (149 - 99) = $50. On September 15th you're billed the $50 on top of the fee for the next billing cycle (September 15 - October 15). Given that on September 15th you're still on the 199 plan it means you'll be billed for the 199 plan, so the total invoice amount will be (199 + 50) = $249.
Now let's say that you didn't want to get billed for the 199 plan, you simply forgot to downgrade beforehand. Check out the next example to find out why it's actually not a problem.
Being on the plan $199 you paid $249 for the September 15 - October 15 period, $50 debt for the previous billing cycle and $199 for the next one. But you know that you don't need the $25k spend limit, $10k would be enough. So you simply downgrade back to the 99 plan straight away. Here is how it'll affect the next invoice amount given that you stay on the 99 plan till October 15th.
Essentially you'll spend the entire month on the 99 plan and that'll be the cost for this month effectively. Which means you overpaid $100 but the thing is that the payment system doesn't just forget about it, it credits $100 to your Revealbot balance and the money doesn't go to waste. The positive balance will reduce the next invoice.
So in this example if you keep the $99 plan for the rest of the billing cycle, on October 15th you're not going to be charged anything because $100 covers the fee of the new billing cycle.
Therefore the best way to save the money is to upgrade and downgrade the plan in a timely fashion so you're never using expensive plans when it's actually not necessary.