The music won't have a license if you do that. You have (at least) two options:
If you sync all of your music to your Zune, you can copy it from backup then erase your Zune and re-sync everything. It will go and get a license for each song.
You can also log in with your Zune Pass account and re-download all of the songs. Your account settings - subscription history has a restore all selection.