How to find a user on Telegram who has changed username?

 

If the user on telegram has changed the username u can't find him directly.

User IDs are not publicly visible by default. Some Clients (like Plus Messenger) allow it anyway.

While Clients don't allow you to find people by their ID only, custom Clients and Libraries for the MTProto API can cache the ID of a user and let you work with those then.

You'll lose the chat history (unless you export it via TDesktop) but you'll get rid of them. If you ever have had a conversation with someone or when you are in a group with someone, you will always be able to start a conversation with that person. You cannot hide your username from your contacts.