Bernina Express in autumn is so beautiful, especially when the larches turned golden and the mountains are already slightly snow-covered; the views are great in autumn, as the air is clear and dry – and the views razor sharp. I have family in the Engadin, I totally love it being there in autumn. It looks like this picture.
Let’s get back to your question: The ticket you are going to book is, in fact, good for any train; you are not going to book a specific train but a ticket for a journey.
Usually, the connection you want to take works fine. I say, usually.
Due to construction works, however, I suggest that you should take the IC 3 train, 557, departure at 06:38 from Zürich HB, just to have some safety margin in Chur. It would mean a pity if you missed the Bernina Express. There is a nice coffee shop in Chur, just opposite the station, if you have time to spare.
Kind regards, and enjoy the ride (think of me, please),