I don’t think narcissistic or abusive men change, or not without a real recognition of the problem, and a huge concerted deliberate effort on their part to change, assisted by people very skilled in that area. It’s super rare. It’s an engrained part of their personality. The type of behaviour you describe maybe falls under this category from the little you’ve said.
I do think that a certain amount of thoughtlessness in a man may change over his lifetime as he learns from previous experiences, and with changing priorities and phases of life etc. Someone who would make a terrible partner at 19 because they are in party mode and at heart they aren’t ready to settle down for example, can therefore make a good partner in their 30s or 40s once they’ve decided they want a serious life partner and maybe a family etc.
Or after losing a partner they really loved due to cheating or taking someone for granted etc, they may resolve to never do that again in a future relationship etc. I don’t think the amount of serious and honest self-evaluation and responsibility-taking in someone who originally behaved poorly in one relationship is all that common, but it has been known to happen on occasion after serious soul searching.
I also think as a pp has said that some men may be terrible partners for some women but an ok partner for someone else. In that a new partner’s needs and wants in a relationship and what they are able to contribute may just happen to line up nicely with the man’s in a way that doesn’t challenge him or cost him a lot of effort. I remember I split up with an ex of mine largely because he was not marriage and children minded (unlike me), and he never prioritised me, instead choosing to hang out with his friends to do extreme sports anytime he had free time, whereas I wanted to spend some time with him alone too when we had free time. He also came across to me as a little emotionally cold, I think largely due to his upbringing (he was sent to boarding school as a young child and I met his extremely cold mother once!) whereas I am a warm and very emotionally driven and open person; I would probably recognise myself in the last poster’s description as being highly sensitive and needing a lot of emotional validation and he definitely wasn’t able to give that to me. Some woman who wasn’t bothered about marriage and kids and who was very into extreme sports herself and is super sociable/extroverted so didn’t mind tagging along and spending all their free time with a big group of people OR who was super independent to the point of being totally happy to spend most of their free time as a couple apart doing different things and rarely seeing him, would have worked fine for him, especially if they were unlike me not particularly sensitive or requiring a lot of overt emotional validation or physical affection. He wasn’t a shit person at heart in my opinion, we just had different life goals and we were incompatible in what we wanted/needed from a relationship and what we were bringing to the table.
I personally don’t really believe this thing that is commonly referenced on mumsnet of men just suddenly finding “the one” and transforming into the perfect man at that stage. I have never seen it in real life at all (if I’m honest I suspect it’s a myth that women who have eg eventually married a previously married man that they had an affair with, tell themselves to make them feel better about the shit people THEY are).
Decent people stay decent people, and shit people stay shit people for the most part. A shit partner for one person may be ok for someone else due to compatibility issues (or as others have pointed out simply lower standards for what a “shit man” is), a bit of maturity can kick in for someone who was a shit partner when younger, what people prioritise in life can change, and very occasionally a shit person (but generally not a narcissistic or abusive person) may do some deep soul searching and change their ways. I personally don’t think this is triggered by meeting “the one”.