Why do you think Grete's attitude toward Gregor changed?

It is in part 3 that Grete first refers to Gregor as "it," refusing now to accept that idea that he is her brother. Grete insists that the "creature" certainly can't understand them, though they've never tried to find out. And she argues that if it were really Gregor, it would have left of his own accord out of consideration for the rest of them. She overlooks, of course, the fact that Gregor has had no way of leaving. She is certain that Gregor is persecuting them and wants to drive them out of the apartment so he can have it to himself, when in fact it is they who have kept him shut up in his room with furniture, garbage, and dirt.

Grete's impassioned speech is the climax of the novel. The conflict, at this point, comes to a close. The family at least realizes that Gregor, who has become the symbol of their alienation, is the problem and has to be gotten rid of. A reversal has taken place. Gregor, in undergoing is metamorphosis, cast aside his duty to his family in order to find his freedom. Now the family wants to cast aside their duty to him in order to find their own freedom.