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No. If this feature were added, it could cause confusion about who is whom. 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. In addition, the website database structure is too complicated to allow easy switching of usernames and it could crash the server if the user has a lot of activity on Scratch. The server code architecture is not designed for name changes.

This has been suggested and declined many times.[1] The practice of creating a new account in order to obtain a new username is known as "account moving". You can remix all your previous projects and invite the new account to all your previous studios and then promote it to manager in all of them. This will move almost all of what was on the old account.

See Also