There was a short time skip after Chungi broke things off with the MC. And unlike other "top" characters in other manhwa, Chungi might have been pushy and a little vindictive but he was also capable of serious introspection.
I don't think he's actually changed, this is how he always was. If he meets with the mc again he may act out out of jealousy again. He is certainly quite self aware of himself and recognises what he does is wrong after but that doesn't mean he wouldn't do it again in a similar situation. He's a flawed person but he is still a good person
Good person being that he treats people well in most situations. Doesn't mean I think he should ever interact with MC again or that he shouldn't change
The side couple is cute and all but I think it's really weird that Chungi suddenly turned a new leaf when he was absolutely pushy with the mc about things he wasn't comefortable with and now he's all cute and nice it just makes no sense to me