In the Catholic faith taking a NEW name, not a “fake” one, is a sign of either new life or a new special calling in life. At baptism we are given a name as we are now a new creation in Christ. At confirmation we take another name (without losing the first one) as a sign that we have now been anointed, set apart, to share in the ministry of Christ as King, Priest and Prophet. Religious men and women, when they profess their vows and are consecrated to God, take a new name as a sign of their new consecrated life- the old self is Gone, they now belong fully to Christ. The Bishop of Rome does the same thing to symbolize that he is no longer just an individual, but is taking up a mantle greater than himself. It isn’t unique to the papacy. You find it throughout Catholicism.