Problem changing short name in SnowLeop

Hi - first post, and new to Mac after years with Windoze.

I bought a used iMac, and started using and installing things, and then realised it'd be good to change the short name.

I've followed the apple guide, (http://support.apple.com/kb/ht1428) but when I log in as root, rename the user directory to the new name and try to then create an account with a matching name, instead of getting the warning / override option the apple guide suggests, I instead get "Name is used by another user" in red, and no option to override.

Any clues?

Thanks in advance.