The leader'll call a Clan meeting.

Leader:"Spirits of StarClan! You know every cat by name. I ask you now to take away the name from the cat that you see before you, for it no longer stands for what (he or she) is. By my authority as Clan leader, and with the approval of our Warrior ancestors, I give this cat a new name. From this moment on (he or she) will be known as (cat's new name,) for (description of reason.)

You don't have to do the, 'description of reason' part.