#usermod -l <newlogin> <oldlogin>
Ex: # usermod -l james john
Where john is the old login and james is the new login id.
Code:
$ man usermod
USERMOD(8) System Management Commands USERMOD(8)
NAME
usermod - modify a user account
SYNOPSIS
usermod [options] LOGIN
DESCRIPTION
The usermod command modifies the system account files to reflect the
changes that are specified on the command line.
OPTIONS
The options which apply to the usermod command are:
-l, --login NEW_LOGIN
The name of the user will be changed from LOGIN to NEW_LOGIN.
Nothing else is changed. In particular, the user's home directory
name should probably be changed manually to reflect the new login
name.
There is more to "$ man usermod" I just posted a bit of relivent info.
Hope that gives you an idea