Is there a way to change the username of an admin account in OS X ?

In one of David Pogue's "Macs for Dummies" series, he has the tip for that. Can't remember which one or where. Try the Help menu also. Altho I am running 10.4.10 and maybe Help has been updated.

Go to: System Preferences => Accounts. From there you can click the plus sign in the bottom right to make a new account (and click the administrator checkbox), or you can change the name of your current admin (though I don't think you can change your "shortname").

thnx. ya i guess it's impossible to change the short name. I ended up creating a new usr.. but the deleting of the old one cause all sorts of permissions malfunctions, and i had to create yet another one to try to fix it.. and all of the old documents still have a big permissions cluster-duck.

Definitely can't change a user shortname. The only way I've seen is what Ben mentioned; creating a new user account and moving everything over. Apple has this (lengthy) guide: http://docs.info.apple.com/article.html?artnum=106824

Regarding your permissions problems...have you tried the following?
-"Get Info" (command-i) on your documents folder
-Change the "Owner" under "Ownership & Permissions > Details" to your new shortname
-Press "Apply to enclosed items"

As always, running a repair permissions under Disk Utility might help.

There is a very easy way to change a user’s shortname. ChangeShortName is a utility that will let you quickly and easily change your shortname. I’ve used it plenty of times with great success.

To change the shortname, you can right click a username in the Accounts pane and then click Advanced Options. But be careful, because every file in your user folder will need to change its path so you can find it again. (as in you lose them)