How can one say that an abusive person will not change in a different relationship?

Abusers are inherently selfish and manipulating . They can manage to present a false image ; but not for long.If the relationship survives any length at all,it will only be a matter of time before he shows his true colors.

One cannot say. Still, past conduct is the best predictor of future conduct. Professional help and intervention can help some. The sad fact is that most abusers are serial.