Resistance to change
There are two reasons that play a role in the resistance to change.
The first one is when we say that something will not happen. In fact this means that a change is forced onto the child. That can bring a lot of disturbance or aggression. The reason is that the autistic child falls into a black hole hearing that, because now he cannot predict what IS going to happen. Even when we explain afterwards, that something very nice is going to happen, the child will not hear it, since he is in a state of fear now. It is much better first to explain what is going to happen, and afterwards what will not take place.
The second reason is that the child needs to avoid a bad feeling. Autistic children often feel unhappy. In order not to feel bad, they actively occupy themselves with feeling good, for instance with rituals. If we interupt them, they resist, because we stopped a pleasant feeling. It is better to go along with the ritual and slowly announce the new occupation.