Don't bother to change rate plans in the middle of the month. Verizon will just overbill you for the rate increase, and assemble a massive penalty for minute overage usage. They say they document this on the change screen. Don't buy it. Don Mead... your customer's don't like your service model!
Backdate the change or future date it. On demand will cause proration. All compaines calculate proration when you change plans when you don't back or future date.
When you backdate you are only charged the difference in the plan.