how do I rename my account name in mountain lion

It is probably an artifact of moving the hard drive or something. There are a number of such messages that can be safely ignored: http://support.apple.com/kb/TS1448

Bless you, etresoft. You definitely deserve Level 7. In fact, I say we bump you to level 8!

I come late to this discussion and I would not add to the confusion, but I would say that this operation may very well be done in the System Preferences, without the need of Terminal.

Simply proceed from a second admin user account. If you do not have one, create one (you can delete it later if you want.)

1. Start with the second admin user account.
2. Go to the Users folder at the root of the system drive and rename, quite normally, the user folder you want to rename.
3. Go to System Preferences -> Accounts, click on the padlock, click the user for which you want to change the name, right click (or CTRL + click) and select "Advanced Options". At this time, change the "short name" (Account name) and the Home directory by choosing the folder that you renamed.
4. Restart.

P.S.

If you need to change the name that appears at the opening session, it must be done directly in the account preferences (full name).

I hope to be well understood as I translated from French.

Problem: My "old_username" home folder can't be renamed. With either of the two methods described in this discussion, when I get to that folder, nothing I do will change that folder name. Am I missing something?

Air 3,1 11-inch, 10.6.8

I have done this operation several times without problem. But sometimes I also encountered the situation you describe.

Unfortunately, I do not know why but usually things go back to normal after some time.

Thank you.

I solved it by using root, which let me do anything.

Changing the Home user name is an oddly difficult thing to do. While there are good reasons for keeping it out of everyday reach, it's also true that Macs often enough pass from one person to another in situations where you don't want to do a fresh install. I have extensive customizations that I want to pass on with the Mac, and doing them all again them from scratch would jave been a big job. So changing the Home user name is a natural thing to do. But absurdly difficult.