how does the title of the work stretch further than just the first sentence to encompass most, if not all, characters all the way until the end ?

Really all the characters change in Gregor's family. Gregor's sister Grete is the only one in the family with whom Gregor was intimate. At first, she takes it upon herself to clean his room and feed him. With time, however, she loses interest in this and leaves him alone. Grete, who first cared so much for Gregor, is also the first to assert that the family must get rid of him. Mr. Samsa, Gregor's father, was content to sit at home and let Gregor work. When Gregor becomes an insect, Mr. Samsa is forced to work. Mr. Samsa feels liberated working and finds new purpose for himself and the family. As Gregor changes so does his family until there is a complete disconnect between the two.