Apple won't let iTunes copy music from one computer to another.
Since you have music on your phone that "presumably" came from another computer, if you sync with a new computer, it will "copy" music to it which it will not permit.
I just went through this last week.
It doesn't "erase" the phone, just the music.
What I did was make sure all the stuff I had on the phone was on the new computer including playlists, then went ahead.
I wish Apple would let me sync the phone with multiple computers like I did with my Palm devices. I kept my address book and calendar synced between my desktop, laptop and pocket device.
Sent from my iPad HD
