Here is a solution. All you need is to get all existing music/videos from your iPod back to your computer and iTunes. Simply use this handy transfer program called Tansee iPod Transfer
http://www.gutensoft.com/Utilities/Tansee-iPod-Transfer.htm
Many of my friends are using it for its easy and safe transfer.
With just a few simple clicks, it will help you copy all videos/songs from iPod back to your selected folder on your computer, and then you can easily add those files back to iTunes.
More importantly, this won't overwrite any original files on your iPod. And all song names are preserved.
In this case, all songs/videos (both new and old) are on your iTunes library that you can syn with whenever you wish to without deleting any files.
Here is step-by-step guide on the whole process:
http://gutensoft.com/Guide/How-to-transfer-songs-and-videos-from-ipod-to-computer.htm
Best luck!
For more iPod support information you can also go to http://www.apple.com