If they were to add Yun-Seong, they wouldn't replace his moveset, but while I'm generally the guy that doesn't see a problem with repeated weapons, I don't think they would add Yun-Seong, because he feels too much like a lesser Hwang. Don't get me wrong, I know they can easily make their movesets distinct enough, but when it comes to making them DLC, it feels like a stretch, DLC characters that had weapons similar to other characters were similar to ones from the base roster (Cassandra-Sophitia and Amy-Raphael), selling Yun-Seong after selling Hwang doesn't even look good to promote, because a lot of people will think they're too similar.
If one of them was in the base roster, I'd absolutely agree that the other could be DLC, but both of them? I don't think so. It doesn't help that Yun-Seong doesn't really have much to add, and storywise, he is kinda similar to Hwang as well, with his relationship with Mi-Na. Algol feels like a much more likely add, he has been teased in the story already, he has potential to set up the events of the next game and his moveset is very unique (despite Azwel covering the theme of using the two swords), so I think he has a better chance to return than Yun-Seong, considering that both can be the "STAR" thing.