It's kind of complicated when you change plans in the middle of a billing cycle. Think of it like this. When you paid last months bill, you were paying for the minutes you have available this month (technically pre-paying your minutes). For example's sake, let's say you changed plans 15 days into the billing cycle. Since you already paid for 30 days worth of minutes on your current plan and are changing that 15 days into it, they will prorate your bill. What this means is that they take your bill amount and divide it by number of days in a month (let's say 30) to get the average daily charge. Since you only used 15 days that you already paid for, they will issue you a credit for 15 days worth of charges. On the flip side, you are adding a new plan. They will do the same thing with that and prorate it's charges, because you will only be using that plan for 15 days as well. So you will also see a charge for 15 days worth of usage on the new plan. Then to top it off, you will also see the full monthly charge for the next months plan.
So, on your next bill, you will see
- a credit for X amount of days you had left in this billing cycle when you switched
- a charge for X amount of days your new plan was active
- a full month charge for the next month of service on your new plan
Clear as mud? Hope that helps.