Update: These cards are disenchantable for full value until January 11. We'll post a reminder on the 9th! (Source)

Great news everyone! A new Hearthstone patch is now live on Battle.net and we've got full dust refunds now available for Drek'Thar and Vanndar Stormpike. Mobile players, your patch will arrive around three hours later at which point you can disenchant these cards for full value.

This arrives a little over a week since Blizzard confirmed via their hotfix notes that we'd be seeing the dust refunds in the future. It was quite a common point of feedback from the community that it didn't feel great to open these legendaries out of card packs when it was clear that you'd eventually be able to get them for free from the legendary honor questline (which for the record, Alliance are currently winning). Celestalon later shared details about how the Hearthstone team originally thought about the legendary cards with the main point being they viewed the cards as non-refund worthy because it would take a long time to get the golden versions and if you opened them up in a pack early on, you'd be getting good value from that card until then.

If you already disenchanted Drek and Vann, no worries! Another update will be going out later today to provide you with the extra arcane dust you lost out on.

Quote From Blizzard

The base versions of Drek’Thar and Vanndar are now eligible for full dust refunds. We will be doing a separate update later on patch day to grant additional Arcane Dust to players who already disenchanted either card.