Ironically, given the title of this story, it is possible to argue that this story does not focus so much on the change in Gregor as in the way that Gregor's state and condition has not changed much at all. Note the way that the newly-transformed Gregor does not spend hardly any time debating or questioning his new condition. He just continues to moan about his position in life and how he is exploited through his work, and although he does not say this, exploited through his family, who are dependent upon him. This indicates that his actual state has not actually changed that much. He was treated like vermin before, and the only difference is that he has now become what he has been treated like.
What has changed though is the physical nature of his appearance. This of course makes mobility much more tricky, as his attempts to get up show:
He therefore first tried to get his upper portion out of the bed, and to do so he cautiously turned his head toward the side of the mattress. This actually proved easy; and eventually, despite its breadth and weight, his body bulk slowly followed the twisting of his head. But when his head was finally looming over the edge of the bed, in teh free air, he was scared of advancing any further in this manner; for if he ultimaely let himself plunge down like this, only an outright miracle would prevent injury to his head.
In addition to the physical problems of negotiating movement with his new body, he finds that he is unable to communicate with other humans, as the speech that he utters comes out as animal sounds. Obviously, as well, the transformation and the way that his family treat him mean that he is able to rest and stay in his room and not get out like he had to when he was working so hard as a salesman.