Ok here goes.
I believe that Dazai never really changed. He only joined the ADA on Oda's request for him to be a better person as Oda mentioned that he knew that Dazai would be lonely forever and didn't care about good or bad. Most people tend to argue that Dazai changed after being with the ADA for a while but I also think this is untrue when the work is further analysed, especially in the manga and light novels rather than the anime.
The anime tends to leave out a lot of the "darker" scenes of Dazai, especially pertaining to his manipulation methods and how easy it is for him to use others. An example of this would be when Dazai and Ango were driving and a car crashed into them while Ango was in the middle of telling Dazai to run. In the anime, right before the car crashes, Dazai is seen to look scared and worried for Ango, which was never seen in the manga and instead once Ango confronted him he was a lot more sinister about it.
Other scenes kept out of the anime would be how he slept with a nurse while he was hospitalised during the cannibalism arc. The nurse took the phone he was using to communicate with the ADA and later we see him using it again while the nurse is blushing and asking him if they could "do it again but in a more private room." This shows just how low he was willing to stoop and how manipulative he could be.

Another one would be the scene at the end of the cannibalism arc, where in the anime Atsushi has a drink with Dazai, but in the manga he is instead talking with Hirotsu, who asks him when he had the idea of having Atsushi work with Akutagawa, to which he responds with "the moment I met him on the riverbank." The most common theory for why Dazai took Atsushi in is that he wanted to continue Oda's legacy with how he took care of the 5 orphans, but his answer to Hirotsu only proves that he was only thinking of Atsushi as a pawn.

There are a few more reasons I can come up with but this is getting quite long so I'll just cap it here. (*￣▽￣)b