Don't change the name of the existing account. Create a new account and make it an admin, then disable the old admin account.
BTW, if you're using an Open Directory server setup (i.e. LDAP), you'll actually have at least four accounts you should replace or change the passwords of: a local admin account, an LDAP admin account, a local root account, and an LDAP root account. The easiest way to get at the two root accounts to reset their passwords is to run Workgroup Manager, and choose View menu > Show System Users, then look for users with an ID of 0. Look in both the Local and /LDAPv3/127.0.0.1 domains (there's a semi-hidden domain selection menu right under the toolbar in WGM).
(Note: don't try to delete/replace either of the root accounts; that would be bad. Just change their passwords.)
If you create a new LDAP admin account, make sure you enable it as an admin both with the "administer this server" checkbox on the Basic tab, and give it "Full" admin capabilities on the Privileges tab.