to the OP, you do realize IF they install heavensward and run the game without choosing start that video is exactly what will play right? if you are so concerned you might want to warn to click/hit start so as not to let the video play because that video will take the place of the End of an Era/A Realm Reborn video we have in the game now.

and I agree with others who posted, if this concerns you so much show them the teaser. https://www.youtube.com/watch?v=68CNzwinqlg and warn them about the opening cinimatic.