I'm not sure they've ever had a situation like this in my lifetime at WDW. My wife and I were discussing this, and we can't think of any comparable situation from the past to help figure out how they're really going to do this.
With Dumbo, they gave a date, changed it, and then the wall was just down all of a sudden. The last time they added something major was EE, and I can't recall how they opened it. I know there was a big press day, but I don't know if they did invite-only previews, or just opened it one day without telling anyone and let guests meander into the queue.
We'll be there Nov 24 - Dec 1....and we'll be rearranging whatever plans need rearranging to ensure we get to experience it!