I’m pretty certain we saw the transformation at the end of BvS when he declares, while smiling!, “this is my world. You are my world” and then knowingly sacrificed himself for it. Remember, earlier in that film Lois tells him the symbol “still means something”, and he replies “on my world it did. But my world doesn’t exist anymore.”
By the end of the film, he has decided that he no longer cares if the world is divided about him; this is his world and he will defend it. By the time we catch up to him in JL, he’s an extension of BvS’s ending; this is his world, and now he’s just damn happy to be alive & a part of it.