United in Stormwind Cards List & Guide
Guys, does anyone know how to try new cards in game? Can we enter with Fireside or do we need any code?
As far as I know you can't play the new cards until the expasion is out. Only streamers with a special account that blizz gives to them can do this.
The only thing you can do before the expasion is out is opening the packs in Fireside (usually 2 or 3 days before).
August 3rd is when the expansion launches. You can use some new card in duels before then.
"Lastly, you’ll be able to find United in Stormwind cards in Duels card buckets after the patch goes live and before the expansion launches." https://playhearthstone.com/en-gb/news/23663246