iphone music upload

I recently purchased a new MacBook Pro and would like to get all the music from my pre existing iphone to the new laptop, but when I go to synch the phone to the computer it says that it will take the data on my itunes of the new computer (which is empty) and replace the data on the phone. Is there a way to do the opposite? Have the data on the iphone transfered to the computer? Any help is appreciated.