Useing nLite to upgrade Windows 2000 to WinXP

[Toasty!]

This my be a silly question, but I can't seem to find an answer. I am a new user to nLite and have made a few custom installs. Over all it works great. I just cant figure out how to use an nLite customization to upgrade an existing version of windows. Am i doing something wrong, or is nLite for making new installs only? Windows unattended does not give enough options for unattended setup. But I really want to Upgrade Windows 2000 Pro to Windows XP pro with all the options I chose. How do i do this?

Thanks,

Toasty