If I sync my phone, will it delete my music from my phone?

My computer recently crashed and I had to re-download the iTunes software. I can't add new music to my phone by dragging it because it says I have to sync it. I don't want to sync it because I'm afraid it will delete my already existing music that's on my phone. If I sync my phone, will it delete the music that's already on my phone? How do I get it so I can put music from this computer onto my phone without hurting what already exists? 😟 I don't want to have to re-download 300 songs. Help?