The changes Affleck envisions are to do with the Batman suit. His sentiments, according to pundits is an indication that he will direct the thriller and puts the many doubts that have characterized the movie to rest.
On the suit issue, Affleck was quoted in an interview with USA Today during the premiere of his latest film Live By Night as saying, "Chief among the challenges of doing Batman, will be finding a suit that's more comfortable."
Comfort is the primary motivation of Affleck's desire to change it. He wants among other things that it should be easy to put on and take off, and as he will play the Batman role, he needs a suit that he can stay in a while also directing other scenes that will not make him sweat, 'crazy or exhausted.
Changing of suits is not uncommon. During the filming of the Dark Knight, Christian Bale's Bruce Wayne asked Lucius Fox for a new suit after finding his suit to be prohibitive when it came to movement. In Christopher Nolan's case, though, the change of suit was for reasons of the story.
Affleck's is simply comfort, and it is highly likely that his wish will be granted. Affleck will be the first actor to write, direct and play the lead role in a Batman movie.