Re: Display Name changed from "COOLCRYPTOVATOR" to "The Cryptovator" | Tnx Theymos
Won't they just use your new nickname?
They are updating but slowly 
You can't. However, I would have personally took this opportunity to create the identical username as many places as I can, even if I aren't going to use them, just to try, and prevent impersonators having the exact match.
You can't prevent them with this tactics, because they will always add just one more letter or change number and add identical image with description to look almost identical to original username.Telegram is notorious for this, but Discord is even worse, and there you can create totally identical username with only ID is different, but you need to change settings to enable visible ID.