Some other says that the carving was altered by the successors of Ramses II (may be Merneptah). It was a common practice among Pharaohs of Egypt to destroy the symbols and glory of their forefathers, in order to showcase themselves. Example is Hatshepsut’s temple at Deir el Bahri, where statues and wall paintings were destroyed by her own son.
The temple of Abydos was build by Seti-I and finished by his son Ramses II, greatest of all Pharaohs. So it is natural that his successors tried to erase the trans of from the mind of common people, and brought their own image at front.