Many Lamanites are converted to the gospel. They are so commited to their new way of life that they disown the name Lamanite and take on a new name which is Anti-Nephi-Lehies. Saul of the New Testiment did the same thing after he was visited by the angel and was converted to the gospel. He wanted to completely forsake his past life and start out new so he changed his name to Paul. Maybe when we receive our new name in the temple it is for the same reason. A symbolic gesture to our Heavenly Father showing our renewed conviction to follow His gospel.