If your iTunes wants to erase everything on the iPod and re-sync, that suggests that either iTunes has changed in some drastic way, or the iPod is being Synced with a different iTunes Library.
- Is this the same iTunes library that you have always used?
- Have you Synced with another Library, or added media to the iPod from another Library, using Manually Manage?
Since this is an iPod Classic, the only things on it must have come through iTunes on a computer. If you have only been using this one iTunes Library to manage your Classic, then a "re-Sync" would simply remove everything from the iPod and put back what it finds in the Library. So providing you have not deleted your Library, the iPod would be filled with the same media as before.
The only exception would be any On-The-Go Playlists created on the iPod that you did not save to the Library. But since On-The-Go Playlists are just lists, all you would need to do is re-create the playlist from memory, or write it down before the Sync and re-create the Playlist from that.