Here is an easy way many of my friends use. That is to use this iPod to iTunes transfer program

http://ipod-tool.com/ipod-software/joboshare-ipod-rip.html

It was designed to help transfer music, videos, photos, playlists, podcasts and more from iPod to computer and iTunes. So you can use it to transfer all music and other stuff from your iPod back to your new iTunes in a direct way. Then all files are on your iTunes, and you can sync smoothly without deleting any files.

This is easy and quick. I recommend you try it.
Free Download to try

Best luck!