If I purchase an upgraded plan annually (to receive the discount you offer) and I want to upgrade to a plan that includes more clients, will you only charge me the difference of the two plans?

Yes, you will only be charged the difference.

We will prorate your cost. So in other words, you will not have to pay for an additional plan, we will just adjust the cost and you will pay the difference in costs between the two plans. For example, if you purchase the base package at $8.33 per month – 15 clients (billed annually =$99.96) and then change to the $16.66 – 30 clients plan (billed annually = $199.90), you would only pay the difference when you upgrade ($99.94).