Yes, if you directly click sync on different computer, it will erase all music on the iPod and replace with the content of your iTunes library. But here is a solution. All you need is to get all existing music/videos from the iPod onto your computer and iTunes. Simply use this popular program called Tansee iPod Transfer http://www.gutensoft.com/Utilities/Tanse… The whole process is easy and safe.
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 songs back to iTunes. See detailed guide.
More importantly, this won't overwrite any original files on the 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-transf…
For more iPod support information you can also go to http://www.apple.com/support/