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No
https://github.com/LemmyNet/lemmy/issues/1872#issuecomment-958943903
Changing the username will break many aspects of federation. For other instances you will look like a completely new user, so you wont be able to edit or delete any old posts/comments. I really dont think its worth all the trouble when you can simply create a new account.
Hmm. Sad.
Should I delete or deactivate this old account? Or just leave it?
What I see most people on the Fediverse (who would normally just change their username if they could) do is to make a new account, then edit the profile info of the old account with a note that you've moved to the new account, with a link to it. That way your old submissions and comments are still around to help others, if anyone wants to contact you later they can, and you still get your changed username. That's what I do, but you can do whatever you want! :)
You can set your display name:
Dunno if that helps.