I wonder why Kim Go Eun didn't reprise her role. Was it because of Goblin? I am sure it was a hard choice for her but I am willing to give Oh Yeon Seo a chance - maybe she will bring some spunk to her portrayal of Hong Seol. Fingers crossed.
I think the point of the whole thing is that many people claimed PHJ was being treated unfairly & ditched aside. It's pointless to change him since he's the reason why the movie exist in first place. Also his agency is co-producing the movie :)