re you don't lose the new music? Anything? Is the sync really two-way, or will the contents of my phone just be replaced by the contents of my computer?
You can also re-download. Since everything is tied to your Live ID, in the Zune desktop software, log in with your Live ID, go to Settings --> Account --> Subscription History. You'll see everything you've downloaded and, if not on the PC you're on, a download button will appear.