The only safe assumption here is that Xiba is replacing Kilik. Don't know where all this Mina crap is coming from. Mina will have her own replacement because she is not the same as Kilik! Geez, have you people even played SCIV?
As for Yun-Seong, Xiba might be replacing Yun in terms of personality, and to a lesser extent design, but thing is, Yun could still return. He just wouldn't be the same as he's aged considerably and will be a lot more mature. That way we still have that spunky young male archetype since Yun will no longer play that role.
And even if Yun isn't returning, it's because he's getting his own replacement too. I mean, someone's gotta wield the Chinese broad sword.