They give you credit for what you paid. Essentially you lose the
DVC discount when upgrading. Without looking up exact figures, the WDW DVC AP is around $450 and the Premier pass is around $750. You would have to pay that ~$300 to upgrade.
By upgrading to the Premier, you gain the Premium AP benefits for both parks. Unlimited admission to all theme parks, waterpark, DisneyQuest, etc. for the duration of the pass. Discounts like 20% off merchandise also apply. However, the expiration date of the Premier will be back-dated to the expiration date of your current WDW AP. Take that into consideration when deciding whether or not to upgrade. The
Disneyland Deluxe AP costs around $370. If you buy one of those new, consider whether the later expiration date will allow you to use it for more than one trip. The Deluxe AP does have blackout dates and the benefits aren't quite as good, but the later expiration date may work in your favor.
