erase iphone and replace with contents of this itunes library

I recently bought a IPhone 6plus, before I got the new iPhone I did a back-up of my old iPhone 5.

After I got new phone I plugged it up & it asked me if I wanted to use this iPhone as a new one or if I wanted to restore from back-up or something along those lines. so I restored the new iPhone from the back-up I had made from my other one so I wouldn't loose my contacts, photos, settings, apps & so on,

but I want to put my ringtones & some new music on the new phone but when I went to sync the tones & music, it said " this phone is synced with another iTunes library do you want to erase this iPhone and replace contents with this library.

so my question is,

if I click the OK and erase & replace, will I loose all of contacts, settings, apps, & so forth.

or does it simply mean if I had any tones or music on the phone it would just replace them.

all of my apps and music is on my new computer. & I just need to get my phone synced with the new computer without loosing all of my contacts, settings, photos and all of that stuff.

if anyone can help me on this I would greatly appreciate it. I don't want to take a chance on loosing everything for just come tones & music.