BudyPress Username Changer Plugin allows users to change their username(the name with which they login). It is simple and useful if you want to give your users the freedom to change their username.
Change Username Screen:-
The User can enter a new username there as shown below
And if the username is available and is not reserved, The username will be changed and the user will be redirected to new url of his profile as shown below
And if the user enters a reserved name(say admin, administrator etc), the username will not be changed, and the user will see an error as shown below
Since the plugin updates username as well as user_nicename, there won't be any issues with author archive or any user page. It is safe to update the user name of admins/super admins as well.
In BuddyPress activity stream, the username and link are inserted statically. So, It will not update the old activity stream. Though, the activity will still belong to user but clicking on the username in activity stream for older link will redirect to homepage. Clicking view will work.