The keyword is 'could' because I've gone on to watch these so called series changing episodes and invariably nothing much actually changes. There may be some surprising event during the episode but by the end it's pretty much the status quo.
I understand that promos have to get you hooked into the next episode however the technique of claiming the next episode could change everything is becoming some what over used. Some shows even make this claim every week.
Imagine if Junior Master Chef's next episode did change everything. All the contestants came back and were told from now on it's a spelling bee. No more cooking, we're going to have you spell the ingredients in your recipes. There's a delivered promise.
Not only that, imagine the outcry from the viewing audience. Change is rarely easy and viewers nearly always have something to say about real change. If your audience isn't at least discussing the change then you probably haven't changed anything.
Much like this blog post really. I don't expect much discussion but then I didn't really set out to change much of anything.
I doubt advertisers and promo creators will really take much notice of me and start thinking of better hooks than 'this is the episode that could change everything'.