Yamashita will be unaffected because like... he has next to zero canon to go off of as it is, I don't feel like taking him even further into the 'headcanon only' sphere >w> So yeah, he's a dumb dumb teenager boy but he'll try his best with the kids?

Kohaku however will go back to a 9 or 10 year old street musician kid. Not sure if from before or after his supposed father died, yet, but I'm thinking from right after?
So like, he'll be a child of dark personality, less aggressive but more cynical than Kohaku is now. Absolutely obsessed with violin already though. Kind of aimless, kind of hopeless and shit.