"Then let me trust you for the both of," Sirius replies, giving him as much of a smile as he can. "There's no telling what might happen, it's true, but I know magic tends to give mere Muggle science a run for its money more often than not. Even if... If you do turn, we'll be able to get you back."

It's not a might or a maybe. Sirius says it as a definitive. He's quite done with watching someone he loves go through years of misery. There's nothing he can do for Remus, especially here, but there's a chance with Kieren. And he's going to take it.