I have recently purchased a new computer and would like to sync my ipod without deleting the songs that are currently on it. I have itunes match and the songs show up in itunes on the new computer

I have recently purchased a new computer and downloaded some new music to it, which appears on the new computer on my itunes account. I have itunes match and when I bring up my account all of my songs show up. However, when I try to sync my ipod it asks if I would like to erase everything on the ipod and only download the new songs; the option is not there to add the new songs, just replace the old songs. It says that it is linked with my old computer and cannot link to the new one (old computer is aspire, new one is asus). Can anyone help?