Hi there Rosalinda... You will never get charged more than you are supposed to... If you changed your plan at the beginning of your billing cycle you will be charged or credited the difference between the 2 plans, the new plan and the old plan.
Current plan: 50$
New plan: 55$
The billing cycle has 30 days... So let's say that you change your plan on day 15, it means that from day one to day 15 you will be charged 25$ for the old plan, and then from day 15 to day 30 you will be charged 27.50$ for the new plan.
So the prorated charges you will pay is only 2.50$ that will be the difference between the 2 plans for the days left on your billing cycle...
Here's a video about the first bill, this will also apply for proration...