Question: When Ethan Hawk takes a new life, does he have to shave his fingerprints/get plastic surgery/new teeth etc. because when he started he looked way different than the present day Ethan.
Question: After they find the body in Asher's apartment, and the cops are securing the scene, Paquette says, "lab results for Acosta's DNA came in...he's clean" How did his DNA clear? It came back as James Acosta?
Answer: As "Costa" was initially a suspect, the police took a DNA sample, which did not match any DNA on file - that was what they meant by "he's clean." It basically means that Asher was very careful at not leaving DNA evidence (other than that of others like Hart to frame them). Presumably the real Costa had no DNA profile linked to his identity on file either, or it could have returned a mismatch and revealed Asher's deception.
Answer: Yes, he must do this to truly feel like the different person he is trying to be. Also, so the police won't suspect anything if his traits don't match his identity.