It's been announced that Skull Kid is an Assist Trophy already. Sakurai found SK suitable as an AT, not a character, and that is not going to change. Even if his thoughts on Skull Kid did, he would have to scrap an Assist Trophy that has already been revealed, and likely has had his animations, AI and attack finished, along with having to spend extra development time to create a moveset for Skull Kid, and having to create animations for him, it's far too late to do all of that at this point. And again, he outright stated Skull Kid is an AT, not a fighter, in the pic where he was first shown. It's clear that Sakurai has made up his mind and has given Skull Kid his role, and it is not going to change.
So no, not everything is subject to change.
Anyway, this discussion should be taken elsewhere.