This question already has an answer here:
I purposely changed my display name, but now I want to change it back to what it was.
I tried to change it back myself, but I got this error message:
Display name may only be changed once every 30 days
NOOOOOOOOOOOOOOOOOO! Fine, prevent people from changing their name every day, but at least let them change it back!
Also, I think you should warn users before they perform an irreversible action!