change "shortname" under accounts?

[silverr]

I went into the system preferences to change my admin name, but the short
name box underneath is greyed out, with the old username.

How do i change the shortname in the accounts under system preferences?

[Padawan]

The original "short name" that is established when the computer is first set up cannot be changed (in fact, it even "warns" you of this while you're creating it).

[NanoBite]

You can't change the short name for a user account after the account has been created. If you need to change the short name, you must delete the account and create a new user account with a new short name.

The above is word for word from the Finder help.

[MacHeadCase]

Just came across this Macworld article, ChangeShortName 1.3:

...There are good reasons Apple has made it difficult to change your short user name, the most important being that as your official Unix account name, changing it involves more than just typing a new name in a field; it requires changes in a number of low-level places and modifications of a number of important files throughout the operating system. Apple provides a set of instructions for changing a short username; however, what those instructions are really doing is creating a new account, transferring your files from the existing account to the new one, and then deleting the existing account. This approach mostly works, but it’s a bit of a pain and it’s not really complete—a number of settings may not get updated this way.

As a result, I often recommend that people simply live with their current short username or, if the account was just created, create another new account with the desired name and then delete the unwanted one. But if you’re determined to actually change the short name of your account, I humbly suggest using ChangeShortName 1.3, which offers major improvements over earlier versions thanks to a lot of hard work by my co-conspirator, James Bucanek. ...

Edited to add:

Please be advised that this still remains a dangerous procedure, the author notes on the ChangeShortName 1.3 webpage.