There's a caveat here: once you receive a card, you'll have it marked for "collected" purposes. So if you disenchant a card you've received, it's still considered "owned." So you can't exploit the refund system that way. There's a great benefit to this though: If you don't like a particular card or class, you can disenchant them and not have to worry about getting them in future packs. This "collected" tag lasts forever once you have collected it.
You have 2 options:
Hope to get lucky and unpack the cards again randomly (like the old system).
Or just re-craft them. You won't lose any dust since you disenchanted them for full value.