Yes, we do pro-rate all upgrades. "Pro-rating" means that we will apply any remaining funds you have on your current account towards the purchase of additional space.
For example, if you have a 200 GB annual plan that was purchased four months prior, you have paid $16.60 per month (200/12=$16.60) or $66.60 in total ($16.60x4=$66.60).
If you then decide to upgrade to a 300 GB annual plan, we will now calculate how much money you have unused on your current account and credit it towards your new purchase. In this example, you have used $66.60 which means you have $133.40 remaining in credit (200-66.60=133.40).
We now subtract your credit from your purchase of the 300 GB annual plan. The $300 minus your remaining credit of $133.40 is $166.60, which is what you will be billed to upgrade this account.