If you are Syncing the iPod with the same Library as previously, then it should not be trying to erase and re-sync the existing content. What the Sync will do however, is:
- transfer any playlists created on the iPod (On-The-Go Playlists) back to iTunes , renaming them and then putting them back on the iPod with their new name
- updating the Play Count (listed in your iTunes Library) of any song played on the iPod
- updating Smart Playlists that have changed as a result of the Sync
- transferring new songs onto your iPod
If this is a different Library to the one used previously, then everything on the iPod will be removed and only what is in the current Library will be put onto the iPod.