So, starting from the 26th, we are protected from >2 duplicates for commons, rares and epics. If I understand how it works correctly, then this is the best time to dust goldens if you have one or two golden copies of a card, but don't have 2 normal copies. Not a super-common situation, but for example:

I have nearly all epics from Classic, missing just 8. I have one regular copy of Cabal Shadow Priest and one golden copy. I have never dusted the golden version (don't really need the dust, and even though Cabal is not a meta card, I just liked to keep the second copy around while I didn't get a second regular one).   

Due to the duplicate protection rules, if I now dust my golden Cabal Shadow priest, I will get it again in the first 9 epics that I get from classic. (If I dust it after the 26th, as I understand it, the card will be counted as 'already obtained' and I will not get it again. I will first get the 8 missing epics and after that  I will just get random epics after because the set is counted as complete.) The only caveat here is when Blizz starts counting cards as 'already obtained'. I assume on the 26th, but I could not find any confirmation of that.

The same is true for commons and rare of course, but I don't have any of those in golden that I don't already have 2 normal copies of...  

As far as I understand the rule, regardless of whether you dust that second copy now or after the rule gets added in, that lets you open another copy in packs. Unless for some reason it’s different than the current rule for legendaries, then the game only counts cards as “already obtained” if you currently have them, not if you opened them at some point then dusted them (opening a legendary and dusting it won’t protect you from opening it again)

Dusting it after the 26th would achieve the same effect, as there's only protections against cards currently in your collection, so if you dust a card you can get it again.

Quote From Meteorite12

As far as I understand the rule, regardless of whether you dust that second copy now or after the rule gets added in, that lets you open another copy in packs. Unless for some reason it’s different than the current rule for legendaries, then the game only counts cards as “already obtained” if you currently have them, not if you opened them at some point then dusted them (opening a legendary and dusting it won’t protect you from opening it again)

I don't think any non-Blizzard employee knows yet but I would guess it's like Meteorite says: that getting the golden copy essentially "spoiled" your chances of guaranteeing 2 non-golden copy through the protection mechanism, and that when you dust it won't have any effect.

But you can always roll the dice, dust it now, and pray the "already obtained' list hasn't been tallied yet.

One side note: you write that Cabal isn't a meta card, but between the new Scarlet Subjugator and the buff to Shadow Madness there's a chance that "MC Priest" will see some play this expansion.  At the least, I know there are several priest players who are chomping at the bit to try it out Day 1.

I assumed that the system looks at your collection when you open a pack and sees which cards you have and which cards you don't. That seems like the only way to handle this without causing player frustration, to be honest. Why hardcode it to just the cards which you've opened?

Edit: You guys are right. "Once this change goes live, you will no longer receive more than two copies of any Common, Rare, or Epic until you have owned two copies of all cards of that rarity in a set. Crafting a card counts towards acquiring it, and any cards you acquire and later disenchant (whether golden or non-golden) count as well." is the official language. 

Welp, that sucks.

me trying to work out if I should dust my epic