New iTunes Library and New iPhone (from old back up) Syncing Problems

[kick26]

I created a new iTunes Library to replace the old one that was ****ed up by Apple Music (replaced album artwork, album names, errors where it couldn't find random music files when they were perfectly fine and in their directory folder, etc.). Then I bought a iPhone 6s to replace my aging and slightly broken 5s. Through iTunes (which was open to my new library), I backed up my 5s and restored the backup onto the 6s minus the music that was on the 5s. I went to add my music from the new library to the 6s, but I then receive the following prompt:

The iPhone “Name iPhone” is synced with another iTunes library on “Name MacBook Pro”. Do you want to erase this iPhone and sync with this iTunes library?
An iPhone can be synced with only one iTunes library at a time. Erasing and syncing replaces the contents of this iPhone with the contents of this iTunes library.
Cancel or Erase and Sync

I'm going to try to find a version of the library from 6 months ago, but I do not know if there is a way to unsync an iPhone from an iTunes library.

 

[kick26]

ok so the prompt is misleading. It doesn't erase the entire phone, only the music (or itunes content) on the phone (which is none), and replaces it with the desired songs.