Originally Posted by
sery
Every so often a patch will feature card changes, altering a card's text or attributes. If the card is craftable, and the change is not a direct upgrade or simple technical adjustment, that card will then be made disenchantable for its full crafting cost, for a short period, usually two weeks. This is designed to allow players who have spent dust crafting the card to disenchant it at no cost, minimising the disruption caused by such changes to existing cards. After the adjustment period ends, the card's disenchanting reward will be restored to its usual amount. This also applies to cards moved to Wild format.
source
i was uncertain too so i checked it
Rag, Sylvanas, etc. don't work like wild cards.
When a card is changed, either for balance reasons or whatever, they get full disenchant value.
When a card is moved to Hall of Fame set, like Ragnaros, you automatically get the dust even without disenchanting, meaning you'll get both the dust AND to keep the card.
That's why making a golden copy of Rag, Sylv, Azure Drake, etc. is a good idea right now.