You can do this one of 2 ways.
1. With iTunes on in the profile that shows all of your music in your nano. Plug your new iPhone in, it may give you a message about being sure. Say yes. Once iTunes recognizes your iPhone. Hit the sync button.
2. Create a different profile while starting iTunes. There was a post about this in
TiPb: The #1 iPhone, iPad, and iPod touch blog a few days ago. Once you are in iTunes with your iPhone plugged in, and no music showing. Goto File add file/or directory and start telling iTunes where your music lives on your computer.
Hope this helps.