Radio Times published the full cast list which includes Matthew. Read more here: https://www.radiotimes.com/movies/downton-abbey-sequel-release-date/
Eagle-eyed fans did notice Matthew Goode, who plays Henry Talbot, was not in the trailer. He has been a hugely successful addition to the fifth and sixth series and many are wondering if he will return.
Not to worry, Goode is in the credits and will be making a short appearance in the film much like he did in the first of the sequel.
Thank you to MWG@matthewwgoode on Twitter for posting this.