Hack is a “dialect” of PHP created by Meta to be more performant and have types, among other features. So its not like they shifted their codebase to an entirely new language.
It certainly markets itself as a completely different language, though you'd probably expect that.
On one hand I'm almost positive that someone who's made the transition from PHP to Hack would point out that it's way more cumbersome to switch than you're making it out to be but on the other I have no personal experience with it and I would assume that set is small enough where we might have issues finding one to inform us here.
