These reshoots have so far included tweaking certain characters and lightening the mood, but they might be a little more extensive than that. A new report from Peter Sciretta from /Film is claiming that Joss Whedon is changing the ending of Justice League and is trying to beef up the character of Steppenwolf.
The original ending of the movie, from what I understand, [Steppenwolf] was kind of playing the Silver Surfer role and Darkseid playing kind of the Galactus role, and the ending of the movie was a cliffhanger, and Justice League 2 was going to be Justice League vs. Darkseid. Darkseid arrives at Earth. This kind of down, dirty ending, and from what I understand, that has been completely removed. The ending ends the movie like a movie, it doesn't leave a huge cliffhanger now.
Originally, Justice League was announced as part one of a two part story, with DC villain Darkseid being set up as the main antagonist of Part Two.Read more at HollywoodReporter