Changing your username will not affect your existing followers, direct messages, or @replies. Your followers will simply see a new username next to your profile photo when you update.
Although, you could just get a new account, but this would mean building up a list of followers from scratch, and you’d lose all your tweets.
So, if you want to change your Twitter handle without losing followers and tweets, follow me through the steps below:
==> Log in to Twitter.com
==> Click on your avatar in the top right corner and click on "Settings".
==> On your account settings page, change the username listed in your username field.
==> Click "Save changes" button at the bottom of the page.
I recommend keeping your old username as a "forwarding page" to your new account - but with zero followers.
This means that as soon as you change the username, you should log out, then create a new Twitter account with the old username, using a new email address. Ensure you get this done before someone else does.
Once you’ve created the new account, fill in your bio and tell people that you have changed your name. Just insert something like, “This is my old username. I’ve updated my username to @NewUsername, so follow me there!”
Thereafter, send a tweet from the old account something like, “Hi guys, I've changed my username to @newUsername, you can follow me there!”
Now if anyone stumbles upon your old profile, they will see the link to your new account!
Below is an example from the old Twitter account of Nigeria President-Elect, Muhammadu Buhari:
If you've decided to change your Twitter username, make sure it’s the right decision for you. And this time, pick a name that will look good five years from now.