Previousely , before extension of more than double of character for display name users can set their display only till 20 character but now users can set their display name till 50 character and can also use emojis on display name. Hence extension is 10 character more than double. The extension in character is similar to improvement made by twitter on extension of for tweet . Now users can publish a tweet till 280 character , which is double than previous characters.
As per the improvement users can change their display name not the username . Username and display name are different to each other . If you do not know about difference between username and display name then I will tell you the difference between username and display name . Display Name is the name of user which setup by user to display and Username is an unique word that is similar to users display name that appears in profile URL . It is usually also called as Twitter handle and is displayed as @XYZ. It's also used for logging in, replies, and Direct Messages while display name is a personal identifier (like a real name) displayed in profile page and used to identify a user.