[aglo]

Is it possible to change the short name of a user in Mac OS X? That field is "grey" in System Preferences > Accounts.

[Krypton]

I'd like to know about this too, as after networking three machines two of them don't like each other (i.e. won't connect) because they have the same short name.

I would imagine OS X would be smart enough to do this without trouble, but backing up would prevent any daft user file loss.

[cybergoober]

I seen reports of varying success by changing the shortname to what you want it to be for all instances of the user in NetInfo Manager then logging in as root and changing the user's home directory name to the shortname you want.

Here's Apple's take on it.