DL does not prorate, also please note that if you upgrade your expireation date will remain the same and the cost is the difference between what you paid and the current cost of the pass you are upgrading to, so let's say you bought your pass when it was $99 and the next one up was $129, at that time it would have been the $30 difference, now it is $169, so it is now $70 that it would cost you.