The User ID can be changed in NWADMIN32...
Highlight the User object and select OBJECT/RENAME, and rename the User object to whatever you want it to be.
Once that is completed, go into the User Object Details, select the GROUPWISE ACCOUNT tab and modify the name of the GroupWise Account.
Also, make sure you rename the Users Home Directory. Once you have done that, go back into the User Object Details, select the ENVIRONMENT tab and update the User Home Directory info.
Good Luck!