Here are the instructions from Apple on changing user short name or home directory name. I've used this on a couple of used MacBooks I've bought on eBay. And remember to go into System Preferences>Sharing and change the Computer Name in there.
I'm not sure how those steps worked for you. Perhaps those used MacBooks had older operating systems. The current 10.7+ will not allow you to change the Home Folder name. You can move the contents of the old folder to a new folder using Terminal. However, doing this will cause you to lose every personal setting, email accounts, browser history and on and on. In fact, when booting up into the old username you are presented with all of the welcome stuff you get when you do a fresh install. All of the files and applications and preferences are still on your disk, they are just no longer associated with your user.