How does the upgrade prorate work?

We will prorate your previous month's subscription and apply that discount to the new month, and restart your rollover date. 

Here's an example: 

Let's say you're on the 8-16 k plan. That runs $120 a month. After a few months of growing and managing your list, you're at 17 k  subscribers. Great! Let's get you a little more room to grow. Our next pricing tier is for up to 25 k, and costs $150 per month. Now, you're on a budget I'm sure, and would like to know the cost before you start charging that card. Let's also say that your rollover date is normally the 30th and it's now the 15th, or halfway into the month. What we'll do is take the unused half of the money you've already paid for this month (since you're done with that subscription) and  apply it to the new subscription that we'll set up for you. We'll charge you $90, and you'll be able to mail to up to 25,000 people until the 15th of next month before we charge you again. 

More of an equation person? Here's what it looks like:

New subscription amt - (unused % of month * previous subscription amt) = First month's subscription (full month, ie, rollover date is moved forward to one month from current date)