When you upgrade your account on a monthly plan, you will initially be charged a prorated amount. On your next billing date, you will start to be charged for the new plan in full.

For example, let's say you are on the Basic plan with a billing date on the 15th of every month. Let's also imagine you choose to upgrade from Basic to the Essentials plan, however, you're doing this on the 28th of the current month so after you've already been billed for the basic plan.
​
When you upgrade your plan on the 28th you'll be charged a prorated amount, meaning you'll only be charged for the 28th - 14th on the new plan. So you won't be charged the full amount of the new plan initially you're only paying for the days you're using it.
​
Then, on the 15th of the next month, you'll start being charged the full monthly rate for the new plan.

Check out the real return on investment of field service software.