I have an original (Naxxramus) Baron Rivendare but recently got the core duplicate. Naxxramus cards can only be used in wild but so can core cards.
…Sooo is it safe to dust my original?
What I gathered when I read the article that they posted about core set swaps is whenever they change out the core set, you aren’t going to be keeping those cards. So it’s up to you if you want to keep your originals. I also am unsure we are allowed to use the new coreset in other gamemodes (like wild, duels, TB, etc). My classic set also lost the golden duplicate original I de’d so I don’t even think classic decks can use the borrowed core set 
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Ah borrowed! That makes sense; I thought it was permanently earned. Glad I asked before pulling the trigger~
Thank you!