Disney's priority is to have the attraction up and running for Opening Day, and be reliable.
So they will focus on operation, test and adjust, and having the state (DOSH) sign off on the attraction.
Then, and only then, depending on the date, will look at possible previews. Using CM's is the easiest way to do test and adjust, as the state will allow it prior to approving the ride.
Now, with the Christmas Season in full force, no way will Disney do something that impacts the parks operation. So more than likely, any type of preview would be in January, and of course, only if the attraction is at 100%