TOLORS wrote:

8.1 Changing usernames or changing display names
If a user could change their username (or even set a certain “nickname” or “display name”), this could be very confusing for the Scratch community, especially for those following that user. In addition, this would make moderation of the website more difficult because it would be harder for the Scratch Team to keep an eye on what a user may be doing. The Scratch Team has allowed username changes only on very rare occasions, such as if the username contains personal information or causes gender dysphoria. Generally, the Scratch Team does not change usernames on request (including for reasons such as “I do not like my current username anymore”).

However, because of the way that usernames are stored on the server, changing the cases of letters in your username (for example, changing @Za-Chary to @ZA-CHARY) is NOT rejected; you can discuss it on this topic.