So hopefully this helps other people searching / watching this thread. I’m not allowed to post links, but you can google the phrase below and see it on hearthpwn and echoed by HS Dev Liv on reddit.
" 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."
This makes sense, as if they nerf a bunch of commons, they don’t want people just reopening the same commons and using the dust to craft other cards, then finally just reopen the commons after the nerf event, and the benefits as mentioned above of disenchanting whatever you don’t want and not getting it again until its a “random” assignment. For example, all cards of a rarity in a set are marked as owned, regardless of if you still have them or not - and now its a random assignment from the pool of possibles in that rarity / set.
So for old packs right now, where you may be missing cards, they weren’t marked as “owned” before (like Estelyen pointed out), so any missing card seems fair game it appears.
But once you “own” the card (through opening or crafting), as well as every other card in that rarity of that set, if you disenchant it you may never open it again - its all random at that point. So trying to go for the golden legendary (as I described above) is dead - disenchanting a legendary could open any other random legendary in the set - possibly even a duplicate of one you have.
I’m going to disenchant a common from my AoL packs after I have them all to try it out, but feel free to point out flaws in the logic. And if I am understanding this all correctly, be careful (definitely with legendary cards at least) on what you dust! You may have to craft it again if you change your mind in the future.
Edit - and as mentioned by BigHugger, with it being all random after the initial marking of “owned”, just pointing out the addition that you won’t specifically open it after you have all others in that rarity.