The rules seem to be, pretty consistently:
- If the card is made worse in any way (i.e. it’s “nerfed”), you can disenchant the card for a full refund. So if a card goes from 5 mana to 6, or from 3 health to 2, or changes from “Deathrattle minions cost 3 less” to “Deathrattle minions cost three less, but not less than 1”–then for two weeks, they’ll let you disenchant the card for the amount of dust it takes to craft it.
- If they make the card unambiguously better (lower mana cost, higher health, add a new keyword), they don’t let you disenchant it for full cost. (If you thought Mountain Giant was worth 400 dust when it had no minion type, why wouldn’t you think it’s worth 400 dust now that it has the “Elemental” minion type? So, no refund.)
- If they move a minion to the Hall of Fame, they give you a full dust refund and you get to keep the card. (That’s what happened with Molten Giant.) It doesn’t matter if the card was buffed or nerfed at the time.