Various people in the Bible received a new name when their status changed (Abram → Abraham; Jacob → Israel). Isaiah also prophesied that God would change his people’s name (Isaiah 62:2; Isaiah 65:15). As those in Pergamum receive new status through their victory (resulting from their rejecting the teachings of Balaam and the Nicolaitans), the Lord would give them a new name indicating their new status: perfect communion with him in the New Jerusalem. Obviously, no one can know the new name written on a given stone, because it can only be learned by the one who receives it.
17 He who has an ear, let him hear what the Spirit says to the churches. To the one who conquers I will give some of the hidden manna, and I will give him a white stone, with a new name written on the stone that no one knows except the one who receives it.’