The idea of users being able to change their usernames has been suggested and declined many times. It is on the list of rejected suggestions. The practice of creating a new account in order to obtain a new username is known as "account moving". One can remix all their previous projects and invite the new account to all the previous studios and then promote it to manager in all of them. This will move almost all of what was on the old account, although the project and account statistics will be lost (e.g. followers). One will still have to get the Scratcher rank for the new account, though, which can interfere with moving projects with cloud variables. However, because of the way that usernames are stored on the server, changing the cases of letters in your username is not rejected. It can be discussed here.
No. The only way to change your username is to open an entirely new account, but please note that data such as projects, studios, followers, and forum posts will not be transferred. If this feature were added, it could cause confusion about who is whom. If a username reveals personal information or if it causes arguments and Flame Wars, you should contact the Scratch Team about getting it changed. The Scratch Team will then transfer all projects, loves, favorites, and other information to a new account with a different username. Very few usernames have ever been changed.