When comic books are adapted for the big screen, it's typical for characters and events to be altered to fit the roughly two-hour movie format. As an example, it would have been difficult for Zack Snyder to introduce the comic book version of Doomsday – with his origins and superpowers – in Batman v Superman: Dawn of Justice without further lengthening the movie.
In regard to Justice League, we could debate about the director's reasons for combining so many storylines into a single movie.
Most recently, the DCEU altered Darkseid and Steppenwolf's familial relationship. Darkseid is Steppenwolf's nephew in the comics, but the ruler of Apokolips becomes the uncle in Justice League. You could say that this is a minor change, especially since we don't know whether we'll see Darkseid in this shared universe (the Flashpoint is coming).
The trailer revealed an additional change. In the comics, to create his legions of Parademons, Darkseid's forces kidnap entire populations, transport them to Apokoliptian locations, and torture and transform them.
The trailer revealed that the Parademons are created in a different manner in the movie. Apparently, the targeted population is instantly infected, and they transform right there on the the battlefield. This could spell serious trouble for the Justice League, since as a battle drags on, and more soldiers become involved, the number of potential Parademons increases.
It remains to be seen how the DCEU superheroes will deal with this kind of global threat. The trailers suggest that they'll take the battle to Steppenwolf's lair, which is completely depopulated (it's Chernobyl, after all). As seen in the trailer, a nuclear furnace there is emitting some sort of spores/unknown particles, and these might be what infects the soldiers in the armies going against Darkseid.
Justice League will be released on November 17.