They avoid making the change at the same time for months. She's tired of having that silly human girl in her mind, he doesn't want to deal with her angst over what's been broken for ages, and they generally can't be together in person without a nasty fight, let alone in each other's heads.
But inevitably, it happens one day in January, when Bella's been at Dartmouth for a semester and things are shifting into a different pattern in Forks (in more ways than one).
It shocks Leah into a dead halt when Jacob's thoughts creep into her head and they're about her - it shocks him, too, but there was no way to know she'd been running through the woods at the moment he'd needed to escape, and there was no way to hide the contents of his mind from her in this state, and there was no way to go back. So he loped off in the same direction he'd originally intended, not trying to shield himself, letting her see just how his perspective had changed since his once-precious Bella ran off with her vampire lover.
He's gone for hours, reluctantly turning back only when the sun begins to set. When he gets home, Billy's out, and she's waiting in his living room.
They have a lot to talk about.