I recently had to get a new laptop, an HP. I had 15,000 songs in an ITunes library. Before the old laptop died, I downloaded the itunes library onto a hard drive. I uploaded the library into a new laptop, and tried to synch it to my ipod classic.

The "other" iTunes Library is the one on your old computer, so as far as iTunes on the new laptop is concerned you are now trying to Sync with a different Library.

If you managed to copy everything (music, videos and tv shows) from your old computer to the new one and it appears and plays in the iTunes Library on the new laptop you can safely synchronise the iPod with the new computer's iTunes library. What will happen is that all the old media is removed from the iPod and only what is in your new library will be added to the iPod. This means that anything on your phone (and from your old iTunes Library) will be lost.

Note that if you have music on your iPod that was purchased directly from the iTunes Store using the iPod itself and that music is not in your iTunes Library, it will be deleted from the iPod but not replaced. So before proceeding, make sure that your iTunes Store purchases are in your new Library. This requires that you sign into the iTunes Store both in iTunes and on your iPod.

What will not happen is that iTunes will delete things in your library because you are trying to synchronise an unknown iPod to it.

If there might be anything on the iPod that isn't in your library see Recover media from an iPod - Apple Community. I'd also recommend backing up the library before that first sync.

tt2