When you made the change, the last day of your cycle was prorated, so you will have a small credit on your next bill for the one day the change was in effect. Then you will have a small charge for the new plan for that one day, and they will probably just about cancel each other out. Whenever making a change like this, always either backdate to the end of the billing period you are in, or future date to the next bill cycle start date (if decreasing allowance, you cannot backdate, only future date or on demand). This way you will avoid any proration and potential overage like this (because seriously, you are going to have a $15 overage charge for one one-thousandth of a gb, which is insane). If your bill cycle ends today, call customer service and have them backdate a change back to your old plan to avoid this, then have the change to the 2 gb plan set to start with the new bill cycle.