If you purchase, say, the Collector's Edition of an expansion, and later upgrade to the Premium edition, are you charged for just the difference in cost, or the full price of the Premium?
You are charged the difference in the cost. If the expansions go on sale, you pay the difference between the sale prices; you don't get credit for having payed the pre-sale price. For instance, let's round the collector's version to $90 and the premium version to $140. If you purchased the collector's version and later upgraded to the premium, you'd pay $140 - $90 = $50, not $140. I can't confirm, but I expect you would not get another one-time claimable item for any of the items which overlap between the 2 versions. If the expansion were on sale for 50% then the collector's version would be $45, and the premium version would be $70. If you upgraded while the expansion was on sale, you might think you should pay nothing or get a refund since you already had $90 in the pot. However, you'd actually pay $70 - $45 = $25, which is 50% off the upgrade price if you had upgraded without a sale.
