Pack Opening Order of Operations

This might be a dumb question, but do packs assign what cards you get in the pack when you receive the pack or the moment you open it?  I mainly ask because I've got a quest right now to receive a Classic pack for watching a friend win.  And there are several other quests to receive packs, plus this week's Brawl pack that we'll get starting tomorrow.  And on top of all that, they just announced a bunch of new Classic cards!

So this all boils down to: Will I have a chance at getting some of those new cards if I wait to open them after they are added to the game?  Or will I not have that chance as I received the packs prior to their inclusion?

The pack's conteint is decided at the time you open it. So, if you wait, you can get the newer cards.

That's what I thought, but wanted to make sure.  Thanks!

I had finally bought last wing of the adventure today and got golden classic pack from it then same question appeared; "Should I wait for new cards to open it ?"

I have opened the forum to open a thread about it so thank you for openning this thread.

I have never waited to open my packs before. There were examples of new cards for classic set, like Pilfer. So maybe someone who waited to open packs can help us.

So what I will ask is, Has anyone ever get Pilfer, Icicle, Call of the Void or Tome of Intellect from a classic pack which is acquired earlier then card's release?

I may be wrong but I assume there will need to be some software patch/update before the new cards are effectively introduced, and may be opened out of a pack.