It could be a depot item that costs around 2000-3000 energy so it can't be overused by the same person.
When used it could send a message to everyone on your friend list and guild saying 'Your friend/guild member has changed their name from [Old name here] to [New name here].' just so everyone still knows who you are.
An alternative could be your old name is displayed when a friend or guild member hovers their mouse over your name on their friend list.
This would be for people who currently dislike their knight's name or just want a change. I priced it based on the new reset star for pets which is priced at which it can't be overused. Both your knight's name and sprite ultimates are important choices and it should cost you to change so the name change should be around the same price as the reset star.
Yeaaaah no...
I don't think this will happen.