I'm in the camp that thinks Phil didn't really change at any point into the grump he is now, so much as he ever so slowly learned what a douchebag like him gets for being a douchebag, and because he's not normal in the least, it made him sad but he accepted his future rather than change so he can possibly have a nicer one.
I've never seen Phil look like he wants to be nice to anybody, let alone the people who watch him today. When I watch old videos of his, he may not be so miserable, he can even come off as fairly normal, but he still has his frequent moments of being a dick. It's what he is, a complete cunt, always has been, and the only thing that's different now is that he knows he can't be social AND be himself, and he hates it. So he just lets loose more than he used to.
Good. Better that than getting away with treating others poorly.