Having duplicates is a bad idea from an admin point of view and generally considered bad practice.
export everything from your current AD regarding the users you wish to change details for. Change the details in the exported file and use something like dsadd on the command line or powershell to import the ammeded details back in.
If you are moving from 2000 to 2003 you are just raising the domain function level. This means you will still have all the same users and user details in AD after you have done this.
To completly change everything use the modify command dsmod. be sure to get the fully qualified names correct though (user=user1,ou=staff,ou=x,domain=y) or the commands will fail.
before raising the functional level take a full back up and test a restore to a mock up of your network. Once you raise a domain function level you cannot undo it. However you can roll back to an earlier backup. Test fully before changing anything. This includes the modification of user details when such a large change will be taking place.
use this for reference