My laptop broke and I got a new one. I downloaded iTunes onto it and decided to restart my iTunes because I didn't have too many songs on my old one. I spent a few days getting about 600 songs onto it. I've just plugged it in thinking it would delete all the existing songs on the iPhone and sync all the new songs onto the phone but nothing happened? It just said "syncing iPhone" and about 2 seconds later it said "ok to disconnect". Please, please help me sync all my new music onto my phone 'cause I spent like a year downloading them all.

HELP?!