I think Warner Bros. has always maintained a "Let the cartoon people do their thing, and we'll do our thing" when it comes to live-action projects.
It's also pretty telling back when Batman and Robin was released, they delayed the direct-to-video animated Sub-Zero movie (which also had Mr. Freeze and Batgirl) several months because Sub-Zero was a better movie than B&R, and WB didn't want to be upstaged by an "inferior" project.
Just to clarify, though, "the cartoon people" are also Warner Bros., just a different division thereof. So it's not WB vs. someone else, but WB's live-action feature unit vs. their animation unit.
Replacing who plays the green lantern is one of the easiest things a "Justice League" movie could do. They have 3 human choices
Actually five, counting Jade and the new guy, Simon Baz.