It's common sense. If you have say a 2 GB data plan and its halfway through the month and you change plans you're only entitled to 1 GB out of the 2. 2 X 1/2 =1. If you start a 4 GB plan then it's common sense that since you'll only have it for half the month you'll only be entitled to 2 GB 4 X 1/2=2. Which part confused you?