" i get a message saying that it's not synce with this library do i want to erase it and synce it to the new library?"
Correct. The ipod touch/ipad/iphone will sycn with one computer at a time. Syncing to another will erase the current content and replace with content from the new computer.
It has always been very basic to always maintain a backup copy of your computer for this very occasion. Use your backup to put everything on the new computer, then sync.