Why does iPod content delete when updated?

Hello, I have an iPod touch, and would like to update the latest software. However, whenever I have previously tried to, all the content on my iPod is deleted. Half of my music is on my old computer, and half on the new one, and I really don't want to buy the old half again! Is there a way of saving the full content on my iPod library without using a disc, and moving it to my new iTunes? PLEASE HELP.