Gregor Samsa may have changed into an insect at the beginning of the book but it is evident as you read through the rest of the story that the rest of him did not change. When something or someone goes through a metamorphosis, there should be an evident change not only in terms of physical appearance but also from the inside.
Gregor may have changed but he still wanted to work in order to provide the money and food that his family needed. In his death as an insect, he realized that his family was already capable of fending for themselves since they cannot rely on him anymore to provide their needs.